You can also get yourself into a target-last-target loop if you switch targets and then start spamming buttons with /assist in them too soon afterwards. Effectively, what happens is you don't wait long enough for the server to acknowledge your targeting change, which causes you to retarget that corpse, which causes your client to report you targeted the corpse, which just happens to time just right and notify the server who acknowledges you've targeted the corpse, which starts the cycle over again. It's annoying when you're trying to quickly dispatch a pack of enemies. Only solution is to retarget and give it a second to "catch up."

Or what Fenril said.