I've had one home made rez have a slow leak onto my hard drives (hard drive water cooling
).
I've had a swiftech pump start to fail when It was mounted on its side and turned all my stuff brown, took the blocks and heatorcore to carwash to clean, trashed the hoses, RMA'ed the pump.
Besides that its just been stupid stuff like spilling a little on the motherboard or video card when filling the system, or condesasion. Just proves that most of the time a little water wont hurt a computer.