...not that kind of gas...

Tank filled yesterday. Drove to work and back (20 miles or so). Got in car this morning and gas gauge shows one bar and range to empty shows 0.

As I drove in to work, the gas gauge literally GAINED two more bars (of 8) as I drove and my range to empty gradually went from 0 to 165 now!

While I'd like this to be the case all the time as I burn through gas, something tells me this is not normal. This is the second time it's happened in the four years I've owned it and the things shared in both cases are that I did not personally fill the tank and am wondering if topping it off makes the gauges go hay-wire.

Anybody else have this happen? Any reasons for the cause?