When Nell is alone in the football field pitching, at the moment Woody comes there are 4 balls near her, but we count only 3 pitches and there are no more balls left in the next general shot.
When Nell's father gives her the gift out on the porch, Nell tucks her hair behind her ear as she's pulling off the box lid. When it cuts to a closer shot of her face, her hair is no longer tucked behind her ear.
Horse's car has Pennsylvania license plates in front and back. Pennsylvania only has rear plates.
Yale "Early Action" decisions are not mailed until mid-December, yet Nell receives hers by Homecoming day - an October or November event at American high schools.
At the end of the movie, Nell takes a "sabbatical" from Yale. The writers presumably meant a "gap year" because students can't take sabbaticals -- a sabbatical is a "a period of paid leave granted to a university teacher or other worker for study or travel, traditionally one year for every seven years worked."
The night Woody and Nell switch bodies, Woody grabs a football off his clock radio and throws it at the light switch on the wall. The football hits the wall just above the light switch, doesn't flip the switch, but the light turns off anyway.
Interviews are not required for Yale applicants, and many students successfully gain admission without ever having one. Nell could have simply canceled hers.
When Woody uses the term "different to", Nell corrects him, saying the grammatically correct thing to say would be "different from". However the English language permits both.
When Nell arrives at her locker after Horse and Woody spray her with a puddle, she yells at woody and walks off, locking the lock on her locker backward.