Fan theories are fun... except when they make no sense, have the flimsiest of evidence to them, blatantly ignores the clear evidences against them, etc etc. What are these bad fan theories you have heard? Mine are
1. Harry Potter series: That Neville is the chosen one, not Harry.
I hate this one. For a pretty simple reason. It originates from the book and... well, I'll just post a passage:
“Then — it might not be me?” said Harry.
“I am afraid,” said Dumbledore slowly, looking as though every word cost him a great effort, “that there is no doubt that it is you.”
“But you said — Neville was born at the end of July too — and his mum and dad —”
“You are forgetting the next part of the prophecy, the final identifying feature of the boy who could vanquish Voldemort. . . . Voldemort himself would ‘mark him as his equal.’ And so he did, Harry. He chose you, not Neville. He gave you the scar that has proved both blessing and curse.”
2. The "It's all in his/her head! Nothing is real!"
This is just plain lazy. So lazy that you could apply it to pretty much every fiction ever written or filmed.