The van door remains open after Debbie slams Rusty's arm in it. When the camera changes to the rear of the van, the door is closed.
When the Griswolds are eating with Audrey and Stone, a cow in the background constantly disappears and reappears.
When the van rolls over, the mirrors are torn from their mounts and hanging on by the adjustment cables. The next day the mirrors are back in place and working.
When Stone comes in to the bedroom to show off his six-pack, Rusty's shirt is open. When he reclines on the bed, it is fully buttoned. When the shot returns to him, it is open again.
When the car is flips over it's turning (from the driver's perspective) clockwise. Yet when the Griswolds are seen in the car the background is turning anticlockwise.
When Rusty enters the cockpit, the oxygen masks hanging from the ceiling are used by passengers and flight attendants, not pilots. The pilots' oxygen masks are larger and located next to the seats.
The college is supposed to be "The University of Memphis" not "Memphis State" and the colors are blue/gray/white, not red.
Kevin complains that he will miss the first week of wrestling practice when the family goes on vacation Memorial Day weekend. Regularly scheduled wrestling seasons go from mid November until mid March for high school and January until March for middle school. Any club wrestling would have started much earlier than Memorial Day weekend as well.
There would be no way of the trucker or Rusty knowing exactly who they were talking to over the CB radio. Even though CB radios are somewhat short range, any vehicle on the highway could have been broadcasting those messages.
When Rusty wakes up the next morning at Audrey and Stone's home, he asked Stone why he gets up so early at 6am but the shadows from the sun shows that it is more like noon.
Debbie talks about the family going to the cabin in Cheboygan, where they have gone for ten years. Sheboygan is in Wisconsin, leading some to believe this is an error. Cheboygan (pronounced the same, leading to the confusion), however, is in Michigan.
As Stone Crandall is telling the Griswolds about a tornado forecast, he mentions he saved 2000 people in Harris County based on his tornado tracking prediction. As the Crandalls live in Plano, Texas, Stone would have worked for one of the Dallas-market TV stations where Harris County (Houston) is not in the viewing area and his forecast would not have been seen by anyone in that area. He could just be telling a tall tale.
When Rusty first notices the blonde girl, her Jeep does not have a top of any kind available if needed during a long journey. The Jeep Wrangler used (1997-2006) can either have a soft top or a hard top. The hard top cannot be taken off and stowed for travel within the vehicle. The soft top is obviously missing along with the large amount of hardware needed to use it, and it is also too large to be stowed. However, it is acceptable in a crazy comedy genre.
The Griswolds are shown waiting in line three or four hours to get on a ride at Wally World. Right before it is their turn, they are told that the park is closing and they can't get on. Amusement parks generally cap lines long before closing time to ensure everyone who waits gets to ride. However, it was required for a comedy effect.
The Griswolds go to Debbie's former university. Class appears to be in session. Universities conclude well before Memorial Day weekend, but they could have been having an end of semester celebration. In addition, most sororities are dry and alcohol would not be served so openly on a campus. This could have been one of the rare sororities who opt out of the suggestion to be alcohol-free, risking getting a fine, probation, or costs to their insurance.
When Kevin is talking on the CB radio, he is clearly shown not actually pressing the button to transmit at all.
When the Griswolds are in Arizona and the Prancer is low on fuel, the odometer shows 33.3 miles, despite driving over 2,000 miles from Chicago.
At around 11:00 minutes the iPhone says it's charging but nothing is plugged in to the iPhone.
The Griswolds are shown in a motel in Holbrook, Arizona when Rusty and Debbie decide to "sneak out" to the Four Corners Monument. Four Corners is over 200 miles from Holbrook.
After getting a ride to San Francisco, the Griswolds are shown crossing the Golden Gate bridge. Since they are going to Southern California's Walley World and recently visited the Four Corners, they are certainly coming from the south, where they would not encounter the Golden Gate bridge, as it is north of San Francisco on US-101.
After the Griswolds leave Clark and Ellen's Bed & Breakfast, they are seen driving north towards the Golden Gate Bridge despite Walley World being in southern California. They would be heading the opposite direction.
When the family leaves home, the camera pans up and we see Chicago's skyline. That vantage point of the skyline would place their home in Lake Michigan.
When leaving on their trip, the Griswolds first drive past The Chicago Theatre and then Wrigley Field. Doing so in that order would have them traveling north. They need to travel south in order to arrive at their first destination in Tennessee.
When the car runs out of gas, why didn't they use the CB radio to call for help?
When visiting Audrey's house, Stone introduces his steer Sebastian, a red/white steer with horns, claiming he's an Angus steer. Red Angus is a known breed, but they (like Black Angus) do not have horns. The steer is most likely a longhorn, and Stone simply isn't very bright.
Adena asks James what happened with "that perv that was hitting on you in Arkansas." The Prancer enters the state line of Arkansas in the next scene when you can see the welcome sign passing overhead.