Even weirder, it seemed to only be in the one thread:
http://x-mencomics.com/xfan/forums/s...&threadid=7577
Take a look through some of the posts after the first one and you'll see \' instead of ' in them.
The first post had also been affected, with the big images' URL having been changed somehow to:
Any ideas where to look for the system doing this so I can see what's up?