In Revenge of the Sith, Anakin tries to prevent the death of Padme - predicted in his force visions.
Upon consulting one of the Jedi Masters, this master advises Anakin to be careful about trying to change the future. The very actions someone may take to prevent one version of the future, might be the very actions that lead to the feature that was meant to be avoided.
Does anybody know, the exact quote and who said it? I always thought it is a beautiful quote and it turned out to be true in that exact case.
If I had to guess, I would say the Master in question was Yoda. The meaning relates to his quote "Difficult to see. Always in motion is the future." in The Empire Strikes Back.