A bunch of people at my school would flick the power input switch on the back of the PCs to 110v (240v native in my country) while they were off and wait for an unsuspecting person to come by and boot it up. It'd obviously go bang and start smoking and they'd freak out not knowing what they did to cause the PC to blow up.

Shit move, but having witnessed the shear horror of someone who thought they caused it to blow up was kinda funny.