Undesigned coincidences are internal markers that highlight signs of authenticity within multiple written accounts, in this case for the biblical gospels. Within this argument separate authors show agreement within the details of the narrative of a kind that is hard to imagine if they were deliberately contrived for the purpose of faking authenticity.

This agreement tends to be subtle but ultimately points to the reality of independent eyewitness testimony. The plausibility of the gospel authors getting this type of background agreement on details right independent of one another is rather implausible.

The biblical gospels (and Acts) are chock full of these kind of internal verifications. Something that is notably missing from the later apocryphal gospel literature that starts to arise in the decades and centuries after Jesus.