Whenever Yuffie is concerned, if you like her antics.
Yuffie: Oh Come on! Aren't you a little concerned? Vincent: Oh... Yuffie.. Long time no see...... How's your head?. Yuffie: Can you at least pretend to be sympathetic!?
Another great one is just after (or before...) your first fight with Nero. Yuffie appears and has a dramatic speech and is trying to sound all cool and awesome. After a short pause, Nero turns around, saying he has to get out of there. Vincent is similarly unimpressed.