Gas tank/fuel system issues

While filling my gas tank recently(while almost empty). Not near as much gas went in as should have gone in. But the pump both shut off AND the fuel gauge inside showed full. My local mechanic thinks my fuel gauge probably just doesnt work and I had more gas than what I was showing when I stopped to fill up. But I have a real hard time believing that based on when I had last filled up and how many miles I had put on since. My biggest concern is whether it is risky to continue to run the vehicle if something strange happened/is going on with the fuel system. Is there something weird that can happen when fueling up involving fumes or pressure or something that could cause the "system" to think its full when it isnt?