The NFL is adding a rule for coin flips this season. The new rule says, "If the coin does not turn over in the air or is compromised in any way," the ref must toss it again. So it has to flip at least once and can't hit anything on the way down.
They're adding the rule after the overtime fiasco that happened during the Packers-Cardinals playoff game last season. The coin didn't flip in the air, quarterback Aaron Rogers freaked out about it and the ref tossed it again. The Cardinals won the second toss too though, so it didn't really matter. Then they scored in overtime and won the game.
You can check out the coin toss fiasco that inspired this new rule below.