Let's take two players, Alice and Bob. Alice has a Tarmogoyf. She asks Bob what card types are in his graveyard. Bob fans out his graveyard, they do the math, and conclude Alice's Tarmogoyf is 5/6.
Alice attacks, Bob blocks, and Alice casts Temur Battle Rage. That turns the Tarmogoyf into a lethal double striking trampler. Bob goes into the tank to see how he can possibly avoid dying, and then he notices that the Tarmogoyf is actually 4/5. In other words they miscounted, and not only does he survive, Alice dies in the counterattack. Oops.
What happens now? Can Alice rewind the attack? If so, what happens to the fact that she revealed she's holding Temur Battle Rage? I imagine the answer to this question depends on what the REL level is; if so I'm interested in all RELs.