Used to be you could do it and get away with it. If you had your boarding passes in advance, sometimes you could present them at the gate and they'd tear them there and stamp the pass. Those miles would credit. You could also have "forgotten something" and walk back out the jetway after it was taken there and usually not be noticed. Tricks like those were most common at high-fare fortress hubs where people would buy a 4 leg flight that connected through that hub and use only 2 passes. Some would have the nerve to even send in the first pass and request credit.

Those days are long gone, though, thanks to all the new tracking and 9/11 (though they were gone some four years or so before 9/11).