Beer glasses
Hardly revolutionary, but often I would fill an empty pint glass with water, place something over the top (a beer mat, or plastic sheet or whatever), invert the glass onto the tabletop and silde away whatever it was I had covered it with. Then I'd leave.

Voila, one upside down pint of water on the table, which is well nigh impossible to remove without spilling the water everywhere.

Also, one day, two mates and I were in Cambridge (can't quite remember why, but there must have been a good reason as it's 400 miles from here) and we did the same, only with wasps instead of water.

Thinking back on it, that was probably quite a bit less effective, as the wasps would simply have flown away. I never was much good at the rebellion thing.
( K2k6 has a proper job these days, Fri 20 Jul 2007, 14:12, Reply)