Continuity mistake: When the family sits down at the table and the camera pans back and forth from Audrey and Rusty to Ellen and Clark Audrey's hair is in front then it's in back and it changes about nine different times.
Other mistake: If you watch closely when the Griswolds approach the limo driver at the airport, you will see a pilot with glasses walk by in the background, and he sticks his tongue out at the camera and looks right at it.
Factual error: On Marty the blackjack dealer's name tag, his hometown is spelled out 'Masellon, OH'. It should be 'Massillon'. (00:56:10)
Other mistake: While touring the dam, Catherine (Ellen's cousin) mentions that Eddie told her that they'll get electricity some day. But earlier, we saw a working refrigerator (with a light that went on and off) in the trailer's kitchenette. When Eddie and Clark are digging for the buried money at night, we also see lights on the trailer's exterior.
Continuity mistake: At Cousin Eddie's "house", right before they go to Hoover Dam, in one scene a noticeable dust storm is happening but in the very next shot, its not.
Visible crew/equipment: When Christie Brinkley is keeping pace with Clark in her sports car, the camera vehicle is reflected in the passenger side window of her car. It's not the Griswold's minivan.
Answer: The sign said $30,000.