Any known problems with fuel intake?

We work on a lot of Toyota Camrys and I have not seen this problem. My first question is could the fuel tank be caved in on the bottom making it take less fuel or can you get 12 gallons or more into the tank when it is actually empty? The "low fuel light" is built into the tank sender unit. The wires at the sender unit in the tank consist of wires for the fuel pump, a ground, a wire to the fuel gauge and a wire that when grounded turns on the "low fuel light". Either the tank is caved in (unlikely especially if you can add 12 gallons or more), the sender unit is defective or the low fuel light wire is shorting to ground intermittently illuminating the lamp.