When Lloyd first calls Diane, he gives 555-1342 as his phone number. But when Diane reads the message taken by her father, the number is 555-2342. Even though she has the wrong number written down, it still works.
When Diane goes out with Lloyd to the party, she doesn't have her yearbook with her. She is reading what people wrote in it after the party in Lloyd's car. When he drops her off in the morning, she doesn't have her yearbook with her.
The key bag at the graduation party alternates in fullness throughout the evening. When it is handed to Lloyd at the beginning of the night, it appears full, and in subsequent shots (which show Lloyd collecting keys) the bag goes from appearing nearly empty to its eventual (and initial) state of being filled.
In the break-up scene in the car the background changes completely. Before the line "You told your dad?" you can see an intersection and a hillside behind them, then after there is a yellow house and two parked cars.
When Lloyd first calls Mr. Court, on the table next to him, there's a photograph of Diane in black and white, talking on the phone. Later on when Diane calls Lloyd back, the photograph has changed to another of Diane being next to some flowers.
Diane's father gets the news of her winning the Reed Scholarship by phone and rushes over to the nursing home to tell her. However Lloyd congratulates her on the phone for winning it the same day, saying he read about it in the paper.
When James Court is shown flirting with the woman at the mall in the checkout line, his credit card was shown being declined. The implication is that his troubles with the IRS (Internal Revenue Service) were responsible for this. However, until the IRS either freezes ones assets or seizes them, then credit card companies will not decline purchases unless the consumer was delinquent or overdrawn on their account.
As shown later in the film, when his attorney is discussing a plea deal with the IRS, they have taken no such action and, in fact, are attempting to obtain assets through forfeiture pursuant to his guilty plea.