I had this idea whilst in a pub so applied it to being within a pub but I suppose it could be used anywhere in public.
Basically it would be a wall mounted LCD / Plasma screen that has a gold border or whatever so that it looks like a mirror.
Mounted in the frame of this would be a web cam connected
to a pc.
It would display a reverse image so as to imitate a mirror for people looking into it - but every now and then it would distort the image slightly, maybe add a moustache to someones face - make someone appear in the reflection behind them.
Then it would dissapear shortly after - making the person think Holy Crap, i gotta stop drinking this dark rum, man.
Another idea was to have it imitate a gold fish tank, then have a piranha appear and carve up all the fish, and a moment later back to normal.