Is it possible to over fill the tank and it leak under the car? My daughter thinks she filled hers too much and when she parked on a incline it caused leakage.

Only if there is something wrong, like a loose fuel pump cover, leaking fuel line fitting or hole in the tank.

Yes, it is possible to leak....my wife was filling up her '96 when gas began spilling all over the ground. Turned out the tank vent that goes from the filler door area to the tank itself is a rubber hose. Over the years it rotted until it began to leak. Easy fix, just replace the rubber hose.