I know Hannibal could have won the battle of Zama, near the 2nd Punic wars end, if the Sacred Band cavalry hadn't been denied to him (taught to me by Eb) or if he had maybe advanced on Rome after Cannae, the Romans could have been beaten.
Plus if the Seleucids had defeated the Romans at Magnesia, then during thier later push to Egypt wouldn't have turned back due to Rome's threatened interventiion. Though i can;t remember if Magnesia came before or after that now i think of it...