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.
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.