If I got into a FTL spaceship, I could go out in space, turn around, come back, and interact with my previous self. I could even tell my past-self that I didn't find anything of value -- and that there's no reason to go at all. Maybe the original me decides not to go, and now there's two copies of myself on Earth.

These are the sort of contradictions that break our understand of causality once you introduce FTL.