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.
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.
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.
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.