Ran into this on Reddit today. Wow.

Echopraxia (bouncing ping pong ball) - [Stumbles via StumbleUpon]

Weird. So weird that I had to look up what echopraxia is.

[wiki] Echopraxia is the involuntary repetition or imitation of the observed movements of another. Even though it is considered a tic, it is a behaviour characteristic of some people with autism, Tourette syndrome, schizophrenia (especially catatonic schizophrenia), some forms of clinical depression and some other neurological disorders.

And now I'm even more confused.