If you have a macro in your DPS rotation that does some kind of "target nearest if no target" then you can get ahead of the client-server sync process due to the macro not being aware yet of the client's manual target change.
[edit] This can also happen if you have a "/startattack" in your macros. I put a /stopmacro [nocombat] sort of thing at the top of all my damage macros to prevent unneeded things from accidentally firing if I'm not actively targeting something I want dead. I have to consciously give the server a half-second or so after I switch targets before I go nuts on the DPS key. If I'm in a target-rich environment, I just faceroll the DPS key and let the auto-target stuff do its thing.
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