While the family is traveling around the country, anywhere. If you look at the outside shots and inside shots through the windows of the RV, you can tell that the scenery is a fake background with plain country side in the middle of nowhere.
Early in the movie there is a scene with Bob speaking with his boss Todd. When the camera is focused on Bob, the back of Todd's head can be seen. In that shot Todd's words don't match up with the motions his jaw is seen making.
The camera focuses on Cassie and Earl "making out" while sitting next to each other, but when the shot goes back to the front of the RV you can see the back of Earl's head. He's suddenly sitting back at the table next to his sister. According to JoJo there was a longer/different kissing scene here which was cut.
When Bob is trying to get back from the meeting, he is stuck in traffic. When the camera moves around the RV to look at the traffic, the left side bedroom window is closed. Yet when the RV is climbing the steep mountain, the left side bedroom window is open.
When chocking the RV Bob chocks only the rear passenger side in Nevada, and hurriedly removes those chocks to avoid the Gornicke's. Leaving Mount Watson RV Park he spins on the rear passenger chocks, hurriedly removes only them, and leaves. Arriving at Alpine Soda we see Bob chocking only the rear passenger wheel. However, as Todd arrives you see the rear driver's wheel is chocked as well.
As Bob Munro holds the Y connector over his head at the dump tank, Howie is kneeling over the tank opening holding the end of the hose. The contents of the RV tank makes it's way up the hose, and some does make it's way down towards the dump tank before Bob hollers to stop. In the next shot Howie tosses aside the hose, but there is nothing in the hose.
Before riding his bicycle down the side of the hills, Bob yells to the others while at the gas station. They do not hear him, yet he can clearly hear Travis telling everyone to get back on the Gornicke bus.
When Bob opens the water filled overhead locker to retrieve his rental agreement the locker is about one foot proud of the other cupboards, including the center one housing the television. However earlier when he tries to demonstrate the television, all three lockers are flush with one another.
When Cassie finds out that the food she starts eating is in fact meat, she runs into the RV to spit it out in the bathroom, but as she enters the bathroom, she looks up in horror as she sees meat hanging down, there is no food in her mouth. Unless she spat in out on the floor or swallowed it when the camera was not on her, this is not possible.
When Bob Monroe and the RV is stuck at the top of a mountain road with all wheels off the ground and he guns the gas pedal the RPM goes up to 6000 rpm yet the speedometer is still at zero .. when you have transmission in gear and wheels turn -regardless of ground contact or not- the speedometer will still register your speed through rotation of rear wheels thru the transmission which is in Gear.
When Robin Williams is sitting on toilet in the RV using his Blackberry for sending a Text message.. the cell phone displays 'NO SIGNAL', yet the cellphone screen shows 5 bars for battery strength and three bars for signal strength ..
In the sequence where we meet the Gornicke's for the first time, Billy's right front tooth is only about a quarter of the way grown in. When he meets Bob in the Bathroom later that night, the same tooth is about halfway grown in. When they leave the next day, it's all the way grown in. Thus, that sequence must have actually taken a couple of weeks or so to shoot.
When the RV is stuck behind the low tractor and they show the line of cars and the RV from far away, you can clearly see that the RV is not real, because there is too much white on the back, there are no windows, and there are no green stripes.
When the RV is shown driving down the two-lane highway at the end of the movie just before the squad car pulls it over, it is in the left/wrong lane with the center yellow paint markings to its right - possibly because the camera vehicle is in the other lane, alongside it, or possibly a flipped shot.