One of my coworkers gets this little tiny twitch right by his mouth when he's getting frustrated.  I seem to be pretty good at making him twitch, usually by not understanding something dumb and trivial (like how certain open source applications are so self-important that they have Linux kernel-like version numbering schemes which have no bearing on actual reality, e.g. version 2.4.1 isn't actually that much newer than version 2.2.3).

Now if I can get his whole face to twitch, that'd be something.