"Only the Sith deal in absolutes" - Obi Wan
This has got to be one of the most broken statements... not only is it an absolute statement in and of itself but also Yoda says to Luke in return of the jedi "Do or do not, there is no try".
I'm sure there are tons more, but those are two off the top of my head.
I've always thought that line was unnecessary, because after he says it, he pulls out his light saber and says something close to, "I will do my duty." Which would have been just fine by itself. But I'm looking at it from a writing point of view, from a philosophical point of view I don't have an opinion.