I did this way back when they used punchcard stock (like how CHI subways work?). ATL-MIA and ATL-FLL boarded at gates next to each other, and would fly whichever was cheaper, yet one week the mistake was made. They took the stock, ran it through, and let me board. Took some persuasion, yet they saw the error in their ways (um... did you do a head count?!), and they paid for ground transport MIA-FLL (had to pick up the car).

Sure it happened all the time, in CVG.