If a post is updated while you're editing, a notification will appear allowing you to reload the post (and cancel your own edits). However, it appears the page still thinks you have pending edits. If you attempt to navigate away from the page, a confirmation dialog will ask you if you are sure you want to leave.
Here's what it looks like in Chrome:
I've confirmed this on SO and MSO.
Proposed solution: When you reload a post that you were editing, clear the