During the "breakup scene", Ronnie's collar is buttoned to the top, then open. It flips a few times throughout the scene.
When Cindy goes to get on the lawnmower with Ronald, she is wearing silver bracelets on her left arm. After she is sitting behind him, the bracelets are on her right arm.
When Big John pulls the beer out with his teeth on New Year's Eve and offers one to Ronnie and Iris, they drink Miller Genuine Draft, in a gold can. In the next shot, all three are drinking Budweiser, in a white can.
During the Ronald/Quint showdown, the same kid (multicolor shirt, teal tank top) walks by them in the same direction twice.
The license plate on Cindy Mancini's car disappears and reappears.
An actor's mark is visible on the floor near Big John during the dance scene.
During the poker game without Ronald, Lester clearly looks into the camera simultaneously when he mentions Cindy. Trained actors never look into the camera.
Striking crew members, with picket signs, are visible during cheerleader practice.
After Cindy, Fran and Barbara finish watching Bobby on TV in his football outfit, Cindy says "Bobby's got a lot on his mind right? Football and everything?" Barbara, who is near the TV after turning it off, replies "Yeah". The crew is clearly reflected in the TV.
When Big John first notices Ronald sitting at their table, a boom mic reflected in the glass.
Ronald tells Cindy that the moon landing was on his birthday, which means his birthday is July 20th, 1969. After he fools around with Patty on the night of the dance, he marks a calendar and says "17 years, 3 months, and 5 days" was a long time to go that far with a girl. The dance was on October 16th, which means his birthday was July 11th. Based on the arrangement of days, he must 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 shit bomb at the front door.