What are undesigned coincidences?

An undesigned coincidence occurs when one account of an event leaves out a bit of information that doesn’t affect the overall picture, but a different account indirectly supplies the missing detail, usually answering some natural question raised by the first.

Ronald Knox wrote ‘Studies in the Literature of Sherlock Holmes’ in which he satirically recorded his study of the stories about the famous detective. In this piece, treating the stories as if they are real, he examines whether the stories were all written by Dr. Watson (vs. a deutero-Watson) and whether they are all genuine.

If there is anything pleasant in criticism, it is finding out what we aren’t meant to find out.  It is the method by which we treat as significant what the author did not mean to be significant, by which we single out as essential what the author regarded as incidental.  …

There is, however, a special fascination in applying this method to Sherlock Holmes, because it is, in a sense, Holmes’s own method.  ‘It has long been an axiom of mine,’ he says, ‘that the little things are infinitely the most important.’

He uses methods similar to the undesigned coincidences (even mentioning them) and ends up deciding that Watson wrote them all, but fabricated some of the stories later in life based on the various inconsistencies in “the little things”.

As to actual inconsistencies.  In the mystery of the ‘Solitary Cyclist’ a marriage is performed with no one present except the happy couple and the officiating clergyman.  In the ‘Scandal in Bohemia’ Holmes, disguised as a loafer, is deliberately called in to give away an unknown bride on the ground that the marriage will not be valid without a witness.  In the ‘Final Problem’, the police secure ‘the whole gang with the exception of Moriarty.’  In the ‘Story of the Empty House’ we hear that they failed to incriminate Colonel Moran.  Professor Moriarty, in the Return is called Professor James Moriarty whereas [we] know from the ‘Final Problem’ that James was really the name of his military brother, who survived him.

Doyle responded to Knox’s study with the following:

I cannot help writing to you to tell you of the amusement- and also the amazement- with which I read your article on Sherlock Holmes. That anyone should spend such pains on such material was what surprised me. Certainly you know a great deal more about it than I do, for the stories have been written in a disconnected (and careless) way without referring back to what had gone before. I am only pleased that you have not found more discrepancies, especially as to dates. Of course, as you seem to have observed, Holmes changed entirely as the stories went on.

This video explores the work of Dr. Tim McGrew  who does a similar study. He explores how each gospel author records otherwise insignificant facts in their account of the feeding of the 5000 that when taken together, unlike in the Holmes study, end up providing good evidence that the gospels contain accurate accounts of the event.

In a comment on a blog post, Dr. McGrew says:

The undesigned coincidences among the gospels provide a cumulative case that at numerous points the authors of the gospels were faithfully and independently reporting actual events rather than merely copying one another or engaging in mythic elaborations.

In the same post he writes:

the interesting thing about this argument is that it is completely independent of the ordering of the synoptics. It matters not one whit whether you take the position of Streeter or of Griesbach or of Wenham or of Lindsey and Bivin. The undesigned coincidences provide evidence for the authenticity of these documents and the veracity of their contents no matter who came first.

You know my methods, Watson: apply them. McGrew certainly applies them here.