I posted an answer and then made several edits to it:
https://math.stackexchange.com/posts/23268/revisions
What's shown in the revision history with the summary "clarification" was actually two separate edits I made -- I added the bulk of the text in an edit that I summarized as something like "added $a$ and $b$ and how to determine them", and then made only a minor modification to that and summarized it as "clarification". That first summary and its diff don't show up in the revision history, and the diff that's shown for the summary "clarification" is actually the diff for both edits combined.
I think this is the first time that I used dollar signs in the edit summary, so the problem might be related to that.