It might also depend on how instinctual the person is at combat. If they can attack without thinking about it then they may automatically parry a swing as they are surprised over the sentence, while somehow who has to think through their movements may be distracted just enough to slip up, given what skallagrim said of the same frame of reference and also how much you personally know them. Obi Wan would be very surprised to hear Anakin say something random but if a random Sith ran up to say it then it would have little effect.

Freaking someone out by pretending to be crazy could in itself make them slip up as well. Maybe I could make a sadist bad guy who does that more to watch them freak out then needing an opening. "I'm going to eat your earlobes!" He whispers into their ear.

P.S. Evvie might just agree with the bizarre statement and think nothing of it, depending on what was said.