If you start writing a comment, but then decide otherwise and leave it there, the comment will get posted upon your leaving the page - but only if the page is being left through entering a new address in the URL. If you click a link, or the "back" button, it won't happen.
Happens in Chrome on OS X. The bug has persisted for months so the version should not matter. Doesn't seem to happen in Firefox and Safari.
Does this have to do with one of these new-fangled URL/history APIs? No idea.
Steps to reproduce:
Start writing a comment somewhere on a SE site. (Tested on Meta and SO.)
Make sure the comment is long enough to be submitted.
Don't submit it, but leave the page by entering a URL in the address bar.
The comment you started will get posted while you leave the page.
This has been bugging me for months and I always thought it was just me not paying attention.
It's not terribly grave, but weird enough. Plus it can lead to awkward situations if a comment gets posted that you didn't want posted after all. Also I'd be curious to learn what's causing it!