Plumbing Fittings Video
Did you play a little weekend plumber? Now you have a leak right where the supply tube goes into the valve. This is a very common location for a leak. If you are using the old fashion chrome plated soft copper supply tube, this can be the cause of the problem. If the bend is not correct, when the supply tube goes into the valve, there will be a very small crack. This will be the cause of the leak.
As an alternative, try on of the plastic supply tubes. They are very flexible. This makes them self-aligning and they seat properly in the valve. Making a leak free seal very time.
To prevent the leaks, be sure the supply line is seated squarely in the valve.