When another player doubles for less..
I too had never heard of being able to make up the difference when someone else doubles for less. After reading this thread, I wanted to try it. I played at Foxwoods in CT a couple nights ago, and someone was doubling their hand for less. I said I would take the rest of the double. The dealer didn't even blink, and he allowed it. Better yet, the hand was a winner. So for the first time, I actually made money on someone else's double down. Definitely will do it again if the opportunity is there (on a correct BS double). There's always the crazy improper doubles that you occasionally see that I wouldn't go anywhere near.