Yaadiyaa!!
@Neal
I was having the issue before I changed to this sig and yes, using smartdark.
I get it maybe every third-fourth post or so. Tried cleaning my cookies, but that didnt help. But like I said, it's no biggie. Only problem it causes is if I am editing a post and then get the error and press the back button on my browser, the edited stuff is reseted so I have to edit my post again.
Most interesting thing is that I started to get this after I changed to firefox 3.6, never had this before. I'll make a test thread to spam to with IE to see if I get the problem with that too.