When Ronald and Kenneth are carrying books down the hall, then Kenneth opens the classroom door, he has a purple book on top of his pile, on the next shot from inside the classroom, he has an orange book on top.
Ronald tells Cindy that we first landed on the moon on his birthday, so that would make his birthday July 20, 1969. However, after fooling around with Patty on the night of the dance, Ronald is seen marking a calendar and saying "17 years, 3 months and 5 days" was a long time to go that far with a girl. The dance was on October 16th so that would make his birthday July 11th, not July 20th. And in fact, based on the arrangement of days, he must actually be looking at a calendar for 1987, not 1986.
When the guys go to egg Kenneth's house on Halloween night, all three of them try to disguise themselves by wearing all black clothing and putting black paint on their faces. However, Big John parks his van right across the street in plain view of the house, and both of them call Ronnie by his real name when they're yelling at him to throw the s*** bomb at the front door.
In the gym dance scene, the actor who plays the African-American nerd who replaces Ronald is seen at one point dancing behind Ronald (presumably as an extra) while he should be in the gym's bleachers with Ronald's former nerd friends.
During the slow song at the dance, you see the lead singer with his back to the audience, then he turns around to sing into the microphone. When it shows Cindy dancing with her date, you can see the lead singer behind them with his back still to the audience.
After their "break-up", Ronnie rides up to Cyndi's house on his bike. When she parks her car, she puts her purse strap on her shoulder. However, when she gets out and closes the door, she puts the purse strap on her shoulder again.
When Cindy, Fran and Barbara are watching Bobby on TV in his football outfit, they finished watching it. Then Cindy says "Bobby's got a lot on his mind right? Football and everything?" Barbara is near the TV after turning it off. She says to Cindy after Cindy's question, "Yeah". In the reflection of the TV, the crew is clearly visible.
In reality there's no way that the kids could have climbed the wall to get into the airplane graveyard. It is located on Davis Monthan Air Force Base and is under constant surveillance. You cannot even stop your car on the side of that stretch of road without military personnel showing up. There also are no dumpsters anywhere near that part of the road. That scene was actually filmed elsewhere in Tucson rather than near the boneyard.
When Big John pulls the beer out with his teeth on New Year's Eve and offers one to Ronnie and Iris, they are drinking a Miller Genuine Draft gold can. However, in the next shot, all three of them are drinking a Budweiser white can.