Why does the revision history show the summary for the previous revision?

Question

This question was edited a bunch of times. If we look at Revision 4, we find something interesting. This revision added a decently sized paragraph, without removing anything, yet the summary says "deleted 242 characters in body":

That seems really odd. At first I thought that maybe there was something in the markdown that was deleted, but the markdown view shows the same thing:

Then I noticed that the previous revision also says "deleted 242 characters in body":

And that revision actually did delete that many characters:

The summary for Revision 4 is clearly incorrect, but it might actually be the summary for Revision 3. But I can't figure out why that summary would be displayed for Revision 4.

I did notice that Revision 4 came a mere six minutes after Revision #3, and was done by the same user. I did some experimenting and confirmed that an additional edit within the five minute grace period by default adopts the summary from the previous edit (and only displays as one revision). But after the five minute grace period it does not default to the previous summary.

So what happened here? Is this a one-time fluke, or is there an explanation for this behavior?

(Other than the user manually inputting "deleted 242 characters in body" into the summary which is quite implausible.)

Answer 1

The user opened the editing form for the fourth revision when the grace period was still in effect, but once they submitted their edit, the grace period had expired.

When opening the edit form while a grace period is in effect, the edit summary field is prefilled with the summary of the last edit. This includes automatically generated summaries. As the user didn't bother to change the edit summary in either edit, the fourth revision got the same summary as the last one.