Plot hole: After Superman has reversed time the Hoover Dam reverts to its previous undamaged state as it should, but Jimmy Olsen should then have been replaced back on the dam. Instead he interrupts the potential kiss between Lois and Superman and complains of being abandoned by Superman in the desert.
Suggested correction: Superman is moving at speeds fast enough to turn back time. He can spend 5 seconds to yank Jimmy away from the top of a dam he knows is going to be destroyed just to be safe in case he can't actually stop it all from happening again.