The drink that the UFO conspiracy theorist carries changes several times before the scene ends.
The cab door is clear, but later sports a large sticker.
"Video Playing Store Security" character gets a haircut and grows a thin moustache while exiting the building.
The taxi cab that Linkletter's character gets into at the Trailways station is not the one he later gets out of near campus. The second one has a different unit number, a No Smoking sign, and a missing whitewall tire.
The guy coming from a funeral bums a cigarette and puts it behind his ear while he smokes another. He stamps that one out before going in the store and the one behind his ear is gone when he gets another out of the pack.
Although the credits list the final song as "Die Graskop Polska", it is actually "Skokiaan".
Decades after this seminal film, Mr. Linklater noted that he mistakenly credited the wrong track from the sourced LP.
In the pap smear scene the street sign for 2nd Street is facing the wrong way.
When Happy-Go-Lucky Guy presses the button to get his change back from the newspaper machine, you clearly see and hear the change falling into the return slot. When he checks to see if it has fallen, he doesn't find anything.
In the dialog between the Ultimate Loser and Stephanie from Dallas (just before the Madonna-Papsmear-Girl arrives) you can briefly see a microphone coming from the top.
Boom mic and pole visible at the top left of the screen when Steve finds out he's not on the guest list.
When "GTO" drops off "True Call", the shadow of the boom mic is visible on the car as it drives past.