Audio problem: In the scene where Tidus and Yuna are conversing at the Farplane, Yuna says something slightly offensive about Tidus' mother, to which he replies, "Whoa there, that's my mother you're talking about!" Almost immediately after, you hear Yuna say, "oh, I'm sorry," but her mouth doesn't move.
Continuity mistake: When Tidus is teaching Yuna how to whistle and says "Put your fingers in your mouth like this," he talks as if he has his fingers already in his mouth for the entire line when he doesn't actually have them in his mouth until he says the last two words.