I was editing a post, and I formatted the code better (markdown hates code with tab characters in it). When I was viewing the revisions (in source side-by-side diff mode), some of the lines were really long, and the red highlight overflowed into the left side.
Screenshot for better explaination:
It's not that big of an issue, just a visual bug that kinda bugs me (no pun intended).
P.S. This also happens when the left side has lines that are too long (whitespace after the code).