Revealing mistake: During the scene when Audrey is having a nightmare about eating too much while on vacation, the nightmare ends with her inflating like a balloon from all the food. Her blouse splits open and down the middle, where the buttons have broken, the dark balloon used for the effect is visible.
Audio problem: In the beginning of the movie when Clark is preparing to light the Bar-B-Q, He squirts charcoal fluid out of a plastic bottle. Then Ellen squirts more fluid onto the coals out of the same plastic bottle. Listen to the sound; it is the distinctive sound of a metal charcoal lighter fluid bottle.
Continuity mistake: Near the end of the film as the Griswolds flight back to the U.S. approaches New York City, Clark points out that the Statue of Liberty is visible below. The Griswolds are all seated facing forward (in the direction of flight) on the left side of the aircraft, and are looking down on Lady Liberty through windows on the left side. When we see the subsequent shot of the statue passing by below, the point of view is from the windows on the right side of the aircraft.
Continuity mistake: In the Italy scene after the visit to the Traveler's Cheque center, the Griswold's decide to go shopping with the $3000 they received. Rusty proclaims "Yeah Dad, we've been in these clothes a long time, I feel rank." Shortly after walking out of the store wearing their new clothes it changes to the scene right before they see Ellen on the billboard, and they are back in their old rank clothes.
Visible crew/equipment: When the Griswolds are in Italy, Ellen sees the billboard advertising her seductive video. She storms away and is later seen backing up into a parking spot at their hotel. The driver is a huge stunt man and notably much larger than Ellen (if not most women) and when parked the petite Ellen exits the vehicle.