Watch when Dante is painting his girlfriend's fingernails and when they come into the conversation on how many people they've had sex with before each other. After Dante finds out that she has had sex with 3 people, INCLUDING him, he asks her, "Who were the other three?" Doesn't make much sense.
Actually, Dangte says "Who were the three? Besides me..." or something to that effect. It definatly was NOT a slip-up though...go watch the movie again, and listen closely.