Right after the car screeches to a halt in front of the parachuting school sign, Hicks takes off his glasses twice.
When Kenneth arrives at the frat house after the graduation ceremony, Phil says that Kenneth needs to be packed and ready to go because he needs to be in Dallas tomorrow night for the wedding. The next day on the Groover's road trip to west Texas, Phil says again that they need to get Kenneth to Dallas by "tomorrow night" for the wedding. On the road trip Phil should have said "tonight."
At the end of the film, when Gardner is on the hill overlooking the wedding ceremony lights going out, the sky is much brighter behind him on the face close ups. When it returns panoramic, it is now early evening again....
In the beginning of the film, when Gardner is throwing darts at a picture, the darts change their order from one shot to another.
Their license plate is torn off with the front of the car, assumed lost forever, and is obviously missing when they arrive at the service station. Later, however, after the repairs, license plates can be seen on front and back.
According to the date shown on the screen at the beginning, the movie takes place in May 1971. One of the first songs we hear at the frat house is Elton John's "Saturday Night's Alright For Fighting", which wasn't released until 1973.
In the first scene when they are making blended drinks in the dorm you can see a container of Citrus Hill concentrated orange juice, the Citrus Hill brand did not come out until 1983.
When Kenneth is leaning out of the car in the back seat to pick up the beer bottle from the road, the wire holding the car door open can be seen. Don't try this at home son, this is movie magic.
During the helicopter/airplane chase in Dallas, the highway exit signs mention Stillwater, apparently indicating that the scene was shot somewhere in Oklahoma.