Why is water leaking from my corner valve?

Turning the angle valve all the way off won't help if it's leaking behind it - the problem's in the fitting itself! Most of the time it's a worn seal, and you can actually replace that yourself if you're a bit handy. You just need a wrench to unscrew the valve, then you swap out the busted rubber washer (costs practically nothing) and screw it back on. If you're not comfortable doing it or the leak's coming from a threaded connection, just call a plumber - it's not expensive and saves you the headache!