I ran into a nickel machine like that at the old Hughes terminal one time while waiting for my plane. After losing a little money in one machine I decided to try the one next to it even though it was making some strange whirring noises. Every time I put a nickel in the 3 play machine it gave me credit for 2 nickels. If I put in 2 it gave me credit for 3 but there was no additional payout for 3 coins so I figured why not take advantage of the 2 for 1 payout. After an hour and a half of feeding it nickels my arm was ready to fall off (that was in the days before the "spin" button) but if I remember correctly I ended up almost $20 ahead.