The place I'm at now, we have a shared Exchange in-box for support issues, we all can read/delete/flag items in it. Since we're a small company, we don't have fancy software for this, so when an item comes in one of us flags it a certain color, then works it. A guy that was there earlier in 2008 but quit to go hang out in Mexico for 3 months came back, and was re-hired (against the recommendations of some of the managers). Almost immediately a few of us noticed things being flagged our color, as if we were going to work them, without us doing it. Emails mysteriously appeared in our folders as if we had done them, things like that. We figured it was him doing it, but there's no way to really prove it without watching him in the act. My manager even chewed me out one day for not doing something that was in my folder, since then I keep all items I work in a private folder only I have access to (and management).
One day everything in the inbox got flagged for me, I cleared the flags, they came back again. I called my manager over and he watched it happen, then he quietly walked to behind the other guy's desk and watched him do it. When he asked what he was doing, he said he was just messing around and playing a joke. Since then they've been keeping a really close eye on him, and he's constantly wandering around, chatting with people and not working, bothering the developers (who work based on tickets, and are paid pretty well, so it's best not to waste half an hour of their time with random stories).
Lucky for him he decided to quit and move to another town with his girlfriend, because I think he was about a week away from getting fired. Before he was hired back everybody in the office worked well together, and we were all really productive. Since he came back though, he's formed this little clique of himself and one or two others, all of whom have had their productivity go way down. I can't wait for him to leave honestly, because it means we get a new employee (who will likely be happy to have a good job and will work hard), and we can go back to having fun in the office without him messing it up.