The most recent work on this subject comes from Lydia McGrew, Hidden in Plain View: Undesigned Coincidences in the Gospels and Acts (2017). I must say that almost all insights from this essay came from Dr. McGrew’s excellent book. Dr. McGrew herself appeals to older works herself from which she draws much of her insights, and are surely worth reading.
What is “interlocking”?
“Interlocking” occurs when two independent sources resolve one another’s historical difficulties without meaning to—what some scholars have called “undesigned coincidences.” When two independent sources confirm one another (without intending to), this supports their veracity, showing that they were both reliably recounting concrete events from different angles.
The birth of Jesus
Why do the religious leaders accuse Jesus of being born “of fornication”? This interlocks with the virgin birth of Christ. John doesn’t mention the virgin birth, but he includes this mention of being born of fornication (Jn. 8:41).
The baptism of Jesus
How does John the Baptist know Jesus is ‘the Son of God’? John the Baptist affirms, “I myself have seen, and have testified that this is the Son of God” (Jn. 1:34). How does John know that Jesus is the “Son of God”?
The Synoptics explain that John heard a voice from heaven, which stated, “This is my beloved Son, with whom I am well-pleased” (Mt. 3:17; cf. Mk. 1:11; Lk. 3:2). John says nothing about this voice from heaven, but only says that John knew Jesus is the Son of God. Why wouldn’t he include this miraculous voice from heaven?
The calling of Peter
Why does Peter “immediately” drop his fishing net when he sees Jesus in Matthew 4:18-20? This account makes more sense when we realize that this wasn’t Jesus’ first encounter with Peter: the two had already met in John 1:40-42.
The calling of John and James
Why are John and James “mending their nets” in Matthew 4:21? This is a throw away comment that seems unimportant. But when we compare it with Luke’s account, we discover that Peter had called his business partners to come help with the large catch of fish. Luke records, “They [Peter and Andrew] signaled to their partners in the other boat for them to come and help them. And they came and filled both of the boats, so that they began to sink” (Lk. 5:7). In other words, James and John’s boat was also overwhelmed with fish, and their nets were presumably ripping as well. This would make perfect sense as to why they were “mending their nets.” However, Matthew only mentions the mending of the nets, while Luke only mentions the large catch. When we put both accounts together, we see an interlocked account.
The feeding of the 5,000
Why does Mark mention the “green grass”? Wouldn’t mentioning the green grass be redundant (see Mk. 6:39)? John explains this indirectly by noting that this event (the feeding of the 5,000) occurred just before the Passover (Jn. 6:4). Interestingly, the grass near this region around the Sea of Galilee is not typically green throughout the year—only in spring—only during the time of the Passover. Thus John explains Mark’s comment about green grass.
Why does Jesus ask Philip where to buy bread? In the account of the feeding of the 5,000, Jesus asks Philip, “Where are we to buy bread, so that these may eat?” (Jn. 6:5). Why does Jesus ask a relatively obscure apostle like Philip, rather than Peter, James, or John? Philip only appears in three places in John’s gospel (Jn. 1:43ff; Jn. 12:21ff; Jn. 14:8ff), so why is he singled out here?
The feeding of the 5,000 occurred near Bethsaida (Lk. 9:10), but John never mentions this. However, John does mention that Philip was from Bethsaida (Jn. 1:43-44; 12:21). McGrew comments, “One can… picture Jesus asking the question in a slightly teasing manner. The fact that Philip was from that vicinity makes the question (and the joke) more pointed. If Philip is from the nearby town, Jesus is in essence saying, ‘Philip, you’re from around here. Where can we get bread for all these people?’”
The sending of the Twelve “two by two”
Why does Matthew group the Twelve in pairs of two? Matthew records the twelve disciples with pairings for each: Peter and Andrew, James and John, Philip and Bartholomew, Thomas and Matthew, James and Thomas, Simon the Zealot and Judas Iscariot (Mt. 10-2-4). However, Mark just lists the Twelve without any pairings between them (Mk. 3:16-19). Is there any significance to these pairings?
Mark explains that Jesus sent the twelve disciples out “two by two” (Mk. 6:7). Matthew doesn’t mention the “two by two” sending, but Mark does. Thus both gospels mention pairings, but in different ways, complementary to one another.
Knowledge of Herod’s private conversations
How could the gospels know what Herod was thinking about Jesus being a resuscitated John the Baptist? Matthew records that Herod spoke about this “to his servants” (Mt. 14:2).
Luke explains that one of Jesus’ disciples was “Joanna… the manager of Herod’s household” (Lk. 8:3 NIV). Thus Luke explains how we could know what was happening behind Herod’s closed doors.
Both John and Mark place Jesus at Bethany before his Triumphal Entry into Jerusalem (Jn. 12:1-2; Mk. 11:1-2). John states that this was “six days before the Passover” (Jn. 12:1), but Mark mentions nothing about “six days.”
Instead, Mark coincides beautifully with John by giving a clear sequential listing of the days leading up to Passover (Mk. 11-14). John doesn’t give a day by day sequence, but instead gives the length of time; while Mark gives the day by day sequence, but lacks the total length of time.
The Last Supper
Why does Jesus wash the disciples’ feet at the Last Supper? John records this event (Jn. 13), but John contains no explanation for why Jesus did this.
When we turn to the gospel of Luke, we discover that the disciples had just been bickering over who was the greatest on this very night (Lk. 22:24), and Jesus corrected them by saying, “Who is greater, the one who reclines at the table or the one who serves? Is it not the one who reclines at the table? But I am among you as the one who serves” (Lk. 22:27).
Again, Luke doesn’t explain the foot washing, and John doesn’t explain the bickering of the disciples. However, once these two stories are placed side by side, they reveal a more robust explanation of the historical event.
The Garden of Gethsemane
Why does Jesus mention the “cup” but not explain it? In John, Jesus asks Peter, “The cup which the Father has given Me, shall I not drink it?” (Jn. 18:11). However, this language of the “cup” doesn’t appear anywhere else in John, and it stands alone without any explanation.
When we turn to the Synoptics, we discover Jesus’ meaning. There we discover that Jesus prayed about the “cup” the very night before in the Garden of Gethsemane (Mt. 26:39-42; Lk. 22:42; Mk. 14:35-36; cf. Mk. 10:38).
Why does Pilate ask if Jesus is the ‘king of the Jews’ if this accusation never appears by the Pharisees? John’s account of the trial of Jesus under Pontius Pilate mentions nothing of the religious leaders’ charges that Jesus was the king of the Jews. We only find a blunt question from Pilate out of the blue in John 18: “Are you the king of the Jews?” (v.33) Where did Pilate come up with this question? John doesn’t explain this to us.
It’s only as we read Luke’s account that we discover that the Jewish leaders were raising this charge to Pilate earlier in the trial (Lk. 23:1-3).
How could Jesus say that his followers wouldn’t fight to make him king, when Peter chopped off the ear of Malchus that very night? Jesus claims that his followers would not fight to make him King Messiah (Jn. 18:36). However, this contradicts the earlier record in John that Peter did fight by chopping off Malchus’ ear (Jn. 18:10). If we only read John, this is a glaring difficulty.
When we read Luke, we find a resolution: Jesus healed the man’s ear after it was chopped off (Lk. 22:47-53). Therefore, Malchus couldn’t be produced as a witness to Pilate, because Jesus had healed him.
Why does Pilate not care that Jesus claims to be the King of the Jews? Pilate asks Jesus, “Are You the King of the Jews?” (Lk. 23:3) Here, Pilate was asking Jesus, “Are you leading a rebellion against Rome? Are you guilty of treason and sedition?”
Jesus calmly replies, “It is as you say” (Lk. 23:4, NASB) or “You have said so” (ESV) or “Yes, it is as you say” (NIV) or “You have said it” (NLT).
Does Pilate have him beaten, tortured, and crucified on the spot? No, Pilate immediately tells the chief priests, “I find no guilt in this man” (Lk. 23:5). How odd! Why would Pilate respond in this way?
John explains this difficulty (Jn. 18:33-38). In John’s account, he explains that Jesus told Pilate, “My kingdom is not of this world. If My kingdom were of this world, then My servants would be fighting so that I would not be handed over to the Jews; but as it is, My kingdom is not of this realm” (Jn. 18:36). Once Pilate discovers that Jesus is no political or military threat, it would make sense for him to absolve Jesus of guilt.
Why do false witnesses claim that Jesus would ‘destroy the Temple’? The witnesses brought against Jesus claimed that he would “destroy this Temple… and in three days [he] will build another” (Mk. 14:58; cf. Mt. 26:60-61; Acts 6:14). Where did these witnesses hear Jesus make this claim? The Synoptics never mention Jesus making this claim.
Only John records this statement from Jesus. In John, Jesus claimed, “Destroy this temple, and in three days I will raise it up” (Jn. 2:19). Of course, Jesus was referring to his bodily resurrection—not the destruction of the literal Temple (Jn. 2:21).
Interestingly, the Synoptics contain the accusation of the witnesses, but omit the verbal statement from Jesus. On the other hand, John contains the verbal statement from Jesus, but omits the accusation. It doesn’t seem that these sources are trying to collude with one another, but are simply writing independent histories that interlock together like a pieces of a jigsaw puzzle.
Furthermore, since this event in John was years prior, it would make sense as to why the witnesses distort Jesus’ statement, and couldn’t give “consistent testimony” (Mk. 14:59).
How did Peter gain access to the high priest’s courtyard? It seems odd that Peter—a Galilean fisherman—could cavalierly walk right into this restricted area (Mk. 14:54, 66). Yet John explains that one of Jesus’ disciples knew the high priest, and let him in (Jn. 18:15-16).
Why does Jesus commit his mother to the care of his beloved disciple, rather than his brothers? Jesus tells his beloved disciple to take care of his mother, Mary (Jn. 19:26-27). In first century Jewish culture, this would be quite odd! The mother would go into the care of her other sons, of which Mary had many.
We solve this problem by realizing that Jesus didn’t trust his brothers, because they weren’t believers in him (Jn. 7:5; Mk. 3:20-21; 6:4). This is not necessarily an undesigned coincidence between gospels, because John records the unbelief of Jesus’ brothers. However, it’s interesting to note.
Restoration of Peter
Why does Jesus repeatedly ask Peter, “Do you love me more than these?” This question in John 21:15-17 sounds odd: Why would Jesus keep asking if Peter loves him more than the other disciples?
When we read Matthew and Mark, we find that this question makes sense in light of Peter vowing to never abandon Christ—even if the other disciples did so (Mt. 26:31-35; Mk. 14:26-30).
Can’t we explain these “coincidences” by claiming that the NT authors colluded and weren’t really independent sources?
Critics repeatedly claim that the four gospels were too oblivious or idiotic to notice the alleged contradictions between themselves. However, when we point to examples of interlocking, they claim that the authors were sly and subtle enough to write these features into their works. So which is it? The critic cannot have it both ways.
Most of these undesigned coincidences are found between the Synoptics and John, which have long be pitted against one another. Lydia McGrew observes, “It is entirely possible that the author of John had read the earlier Gospels. He may have had access to them while writing his own work. But this fact does very little to help a skeptic to account for the undesigned coincidences I have noted here. Think what a subtle and almost pointless form of deception it would be for the author of a non-factual book of John to leave out information in his own account, to raise questions by his own somewhat incomplete stories, in order that his stories might appear truthful because a really alert reader might find the explanations in earlier Gospels. That would be an extremely strange form of fakery. Many readers will not notice such coincidences at all.”
These are omni-directional. They all explain one another.
Lydia McGrew, Hidden in Plain View: Undesigned Coincidences in the Gospels and Acts (Chillicothe, OH: DeWard Publishing Co., 2017).
Timothy McGrew, “Undesigned Coincidences” (parts 1-6).
Timothy McGrew versus Bart Ehrman, “Do Undesigned Coincidences Confirm the Gospels?” Unbelievable on UK Premier Christian Radio (Mar. 17, 2016).
John James Blunt, Undesigned Coincidences in the Writings Both of the Old and New Testament (1847).
William Paley, Horae Paulinae (1790).
 Lydia McGrew appeals to John James Blunt, Undesigned Coincidences in the Writings Both of the Old and New Testament (1847), William Paley, Horae Paulinae (1790), as well as the work of her husband, Dr. Timothy McGrew, “Undesigned Coincidences” (parts 1-6).
 See D.A. Carson, The Gospel According to John (Grand Rapids, MI: Inter-Varsity Press, 1991), 51-55.
 Lydia McGrew, Hidden in Plain View: Undesigned Coincidences in the Gospels and Acts (Chillicothe, OH: DeWard Publishing Co., 2017), Kindle Location 1552.
 Lydia McGrew, Hidden in Plain View: Undesigned Coincidences in the Gospels and Acts (Chillicothe, OH: DeWard Publishing Co., 2017), Kindle Locations 880-885.