Sometimes on SMP, XP orbs will randomly start to float around your head until the server restarts. I think this has something to do with the angle that the orbs are collected from, however I'm not sure about that. Tested on vanilla server client 1.4.7, also tested on Bukkit. At the moment, I can't give reproductible steps, however, it happens more often when you are collecting a lot of orbs at once from mob farms. I know they're supposed to float around you, the problem is that they don't collect. When I am experiencing this issue, other players on the server have also experienced the same issue.