If you remember Snape's ramblings on prophecy, prophicies are spoken to those with the power to "fulfill or avert them". The thing is, if they are averted, that might just delay it. If Hero 1 fails to take down Dark Lady Example, Hero 2 might try in a decade and succeed. If Hero 1 failed and thereby averted a prophecy he previously heard, does the prophecy latch onto Hero 2? If it does, does the text and memory of the prophecy itself change? Is there just a new prophecy?

This sounds impossible until you realize that we already have proof of atemporal causation in this world.