Cousin Vicki: I'm going steady, and I French kiss.
Audrey Griswold: So? Everybody does that.
Cousin Vicki: Yeah, but Daddy says I'm the best at it.
In the early part of the film, Beverly D'Angelo and Chevy Chase check into the hotel/motel. She goes in to take a shower but he barges in. If you look real closely, she is wearing underpants and is not nude although she is taking a shower.
Not a mistake but a "before they were famous": little cousin Vicky with the blonde hair is Elaine on Ally McBeal (Jane Krakowski).