In Clash of the Titans (2010), the witches prophesized that Perseus would die, but he clearly didn't... Were they lying in attempt to make his allies abandon him, or were they just mistaken? Was there some other motive for a deception?
Here is what they actually said:
Your journey does not end well.
Fate has spoken.
You will die, son of Zeus!
Written as history.
You will die.
The obvious question is: what is his journey? It may very well extend beyond the movie and, as we know today, it did: we had a sequel. So, the propecy may still be true, but are yet to see it fulfilled (although, knowing Hollywood, they'll never make an end to the journey, unless Nolan takes over ;-)).
Even more, if "his journey" is his life, than the prophecy is completely void of any (useful) information, as it only says that he will day when his life ends. This would be in accordance with the very famous -- and also uninformative -- Pythia's "Go, return not die in war" (see the intro here), and other double-meaning old Greek prophecies.
Addition (inspired by Liath's comment)
I wrote the above under the assumption that Perseus will die, forgetting that he is a demigod and has a choice. I'm not sure if all demigods got that choice; probably not, but he got it. Here is a part of his conversation with Zeus, in the last scene of the movie:
Zeus: I don't suppose you'll reconsider my offer to take your place as one of us.
Perseus: I've got everything I need right here.
And, right away, a few more confirmations, from Zeus' farewell words:
Zeus: You may not want to be a god, Perseus, but after feats like yours, men will worship you. Be good to them. Be better than we were. And if you insist on continuing this mundane human existence, I won't have you do it alone. You're the son of Zeus, after all.
So, Perseus has chosen to live a human life, which in itself may be considered a fulfillment of the prophecy.