It rains and it pours when you're out on your own. If I crash on the couch can I sleep in my clothes? 'Cause I spent the night dancing, I'm drunk I suppose, and if it looks like I'm laughing, I'm really just asking to leave.
I have reproduced the error and it is caused by saving PMs. The site always assumed that the PM was unread (even if it was). The read counter was decremented and caused an overflow.
Saving PMs now check both counts. This should be fixed now.
I'll update everyone's PM count in a few.
2014 is going to be a good year. More content, more streamlining. Be a part of history!