it is a threading bug. if you click the karma button multiple times, you send multiple requests to the server. Each of which first adds the karma, and then "locks" so that you cannot give karma to the same post, person, etc.
if more than one "add" hits the server before the first add does the "lock" you get multiple hits.
i used to think the board coders were dipshits, then i wrote some thin client personal trainer software. Then, i checked the log and noticed that a trainer had done 11 sessions with the same client at the same time. blah.