This happens when someone manually crafts a transaction. In Bitcoin, if you have an input of 100 BTC and make a transaction with a single output of 20 BTC, the

This happens when someone manually crafts a transaction. In Bitcoin, if you have an input of 100 BTC and make a transaction with a single output of 20 BTC, the remaining 80 BTC are kept by the miner. There's no wallet software that would let you do this. It's like paying a $20 check with a $100 bill and saying "keep the change."