In theory, the airline could bill the fare difference between the cost of a one-way ticket to FLL and a one-way ticket to ATL to the credit card used to pay for your ticket. In practice, that doesn't happen.

If you've managed to find cheap one-way fares, that's pretty sweet. Your biggest problem would be the fact that your checked luggage will continue on to FLL, so you also need to have only carry-on.

I take that back....your bigger problem might actually be getting home. If you book a one-way from FLL to whereever, via ATL, you will not be able to board your flight home if you don't board your flight in FLL.