Prophecies are best when they're not too direct, and the twist isn't too obvious if it's there (eg; "no man can defeat me"). Be cryptic, sometimes be painfully confusing if you must, even if the answer is there. The good guy will defeat the villain with an egg? "The feathered one's continuation must be sought", for example, where you have to first figure out what that means, but you're also not told it'll be used, just that it needs to be part of the story. It's a good excercise for Show Don't Tell.
I think the best example of a garbage prophecy is Tim Burton's Alice in Wonderland. Right at the start, Alice is told she'll slay the Jabberwocky with armor and the vorpal blade. She spends 90% of the movie sure it won't happen or she isn't the right person, but at no point anyone implies the scroll of prophecy could be wrong, or vague; no, it literally lists things that'll happen. If the movie was about the journey, that'd be fine, but they go back to it so many times it's like they want you to never be surprised.
A good example is actually a classic, Oedipus. At every turn, he runs away from the prophecy, only for actions to fall into place because he was expecting an obvious event but it's actually things he didn't know before. You know it'll get to the end and be true, but the piling consequences of each thing he didn't pay attention to makes that story interesting.
Personal Example from Splitting Image
The prophecy/guidance in my comic is that, in what was meant to be Mortimer and Dylan's dual crowning cerimony, he recieved instead from the priestess a firm, and clear "You're not meant to rule. You're needed elsewhere.". This flips his world upside down, but it doesn't actually say what's going to happen at any point, just what won't happen. And, in the backstory, that's his villain motivation, because he was told by the gods themselves that he shouldn't use the crown's power, but all his life he was told he was meant for it, as well as a sense of purpose to use its magic to improve the kingdom. If he can't get the crown legit, he needed to get the next best thing... Black magic.