Revealing mistake: When they show the bus with the college girls going by, as they pan over the driver, you can tell there is no door on the passenger side. You can see all the way to the ground and road of, what appears was a blue screen background added. This is made even more obvious by the fact the girl driver is not wearing a seat belt, is supposedly flying down the highway, and if this really was a huge hole in the side of the bus, it would be extremely dangerous as she could fall out. There also seems to be some sort of white mattress pad right next to the driver. (00:14:05)
Revealing mistake: When Jack tells Greg to throw the brick at the RV, when he picks it up you can see it's some kind of spongy material.
Revealing mistake: When LJ has his hands stuck to the bottle, you can see a material actually holding his hands on the bottle.