The thing that I hate the most is associated with time travel and is called an ontological paradox [http://en.wikipedia.org/wiki/Ontological_paradox], or causal loop. It happens when someone or something travels back in time and does something that causes some circumstance that was already present in that person's future, the very future they come from, or even their own existence. The most blatant examples that I know of are in the Terminator franchise. As such, I hate that franchise and almost any non-comedic time travel story. I say non-comedic because I have a rule that I will allow almost anything if it is funny enough. For example, I like the Bill and Ted movies and Back to the Future series, so they are exempt from this rule.