We say "what goes around comes around" when it seems like someone is getting away with doing something unethical without consequences. The phrase can also be applied when someone does a good deed without reward, to indicate that eventually someone will do a good deed for them, too, and their good karma will return to them.
My own feeling about this is that our good and bad deeds might or might not come back to us in our external lives, but that "what goes around comes around" applies more accurately to a cycle of thoughts and actions that happens constantly inside each of us. I chose a quote about this inner cycle from the Upanishads –
"As your desire is, so is your will.
As your will is, so is your deed.
As your deed is, so is your destiny."