Stefan takes his plaid shirt off twice in the locker room before practice.
When Stefan goes to the field to ask the coach to play in the game, his hair is blowing in the wind. In the reverse angle, his hair is laying down flat.
When Stef is called for pass interference in the big game, the public address announces the penalty in real time, before the referee has even made the call, which would never happen.
When Stefan signs the contract to play football for Cal Poly in exchange for a full scholarship, Vern Nickerson's name is the only name on the contract and Stefan signs the Notary Public line, with no Notary Public in sight. The "County" on the contract is a correct Pennsylvania county (Cambria) but the contract also says "State of California" when they are obviously in Pennsylvania.
Cruise is thrown off the team, but somehow his father and brother know about it in their hometown bar moments after the game is over; yet the team hasn't come home yet.
In the big game, the field has turned into a total quagmire in the 2 minutes between when it started (after Stef scores) and the ensuing kickoff. The previously pristine field would not deteriorate that fast.
At the end of the film, Tom Cruise's father and brother are standing next to him when he signs the college football scholarship. A second later as Cruise hugs his girlfriend to celebrate, you see his father and brother walking out of the factory no where near him.
Pennsylvania high schools are grouped according to size of a school's student body. This is to avoid small schools having to play much larger one. Clearly, Ampipe is a much smaller school than Walnut Heights so they would not have ever played each other in football.
The industry appears to be far western Pennsylvania west of Pittsburgh around Ambridge, thus likely the Ampipe name, and they even mention nearby Allequippa, but then they have multiple references to Cambia County, which is far east of Pittsburgh. To their credit, filming was done in Johnstown, which is closer to Cambria County, but the city references are still much farther west.
Before the big game, Stef is banging the back of his head against a brick wall. It would not make the "hollow" sound that is heard.
Ampipe is a small high school with a football team that has less than 40 players. It is therefore highly unlikely that numerous players received scholarships from Division One football programs.
Stef's brother Greg gets fired from the mill. At the end of the movie, he is shown working there with no explanation as to how he got his job back.