While my colleague was distracted…
I put a basketball on his chair. I thought he’d try to sit back down, find an obstruction, and that would be the limit of the ’amusement’. Instead, he bounced right up in the air, like Sonic bopping a Crabmeat.

As a short, compressed screamlet caught in his throat, his arse caught the back of the seat, which swivelled around, making the whole chair s-l-o-w-l-y tip up onto one castor, before toppling entirely against a radiator. My friend ended up trapped twixt chair and wall like a foldaway bed, his knees around his ears, his specs comically askew and his eyes having a strange blank look I can only describe as ‘Does not compute’. We had to turn him off and on again before he even registered that he was on the floor.

I still laugh out loud every time I even think about it (15 years later) and wish I had the words to do it justice, especially the final image. If anyone is familiar with Flaming Carrot, the panel in ‘What are they doing in the Hyacinth House?’ where the alien gets caught up in the ‘click clack chair’ is the closest analogue I have yet found.
(electricteeth, Tue 7 Oct 2014, 13:44, Reply)