leaking toilet

i have an old toilet that was leaking, but because it was seldom used i simply turned of the water at the supply line and drained it. i finally decided to replace its inner workings but when i loosened the nut at the supply line quite a bit of water started pouring out. i managed to slow the leak but it still continues. should water be coming up through the supply line even though its turned off at the valve? is this common? how do i fix this problem? any input would be appreciated. eugene.

Two things to look at. First, when you drained the tank, did you sponge out the water that always remains after flushing? If not, this water will drain from the supply line when you loosen it. If the tank is dry, then I'd suspect a bad cut off valve on the supply line. Shut off the main water supply, install a new valve, and see if that doesn't fix the problem. "New" water should not come into the tank with the valve off.