At Debi's house, Martin's baseball cap is taken off and put on one of her bed posts. After they do the "airplane" and he is leaving, it's in his back pocket even though no one took it down.
In the beginning Mr. Grocer comes out of the hotel and shoots everyone. He has two revolvers which contain six shots each but he fires more than 20 shots without reloading.
Despite never turning around nor leaving the main street of Grosse Pointe, Martin passes the same stores twice.
When Martin first stops in front of the convenience store the shadow from his car shows it is about noon and it is an asphalt street. In the next shot, the front of the store, the shadows indicate a much later time and the street is full of mud and gravel.
While in the bar the lime slice in Martin's drink keeps switching from one side of the glass to the other side of the glass even though he never touches the glass the whole time.
Debi's radio station is FM 79.5. In the US, the FM band is 87.9 to 107.9. This was done to avoid identification with any actual radio stations in the area.
When DJ-ing, Debbie plays a Specials LP with the 2 Tone Label on it. The song she played, Pressure Drop (originally done by Toots and the Maytals), never appeared on any Specials LPs on the 2 Tone Label, but on an album released in 1996 called "Today's Specials" on Kuff Records.
During the reunion party, Martin takes a photo of Debi holding the baby. He is using a Minox "spy" camera which has no flash, but there is a flash when the photo is taken, as well as the sound of a motor drive that the camera doesn't have either.
In the kitchen at Debbie's house, Blank and Grocer both empty their pistols and the slides lock back. Then they both pull the triggers and all 4 guns "click". Neither the Glock 17s nor the Colt Mustangs can click when the slides are locked back.
When Martin is in his hotel room after the reunion, he hears someone knocking on the door. Since he does not know who it is, he grabs a handgun and there is the sound of a handgun hammer cocking. The gun he has is a Glock, which has no hammer to cock.
Martin shuts the Venetian blinds at the radio station, but there is an open set when he sits back down. Those blinds are on a different window.
After Debi announces on her radio show that she's playing an all-'80s weekend in honor of the reunion, the next song she plays is "Rudie Can't Fail" by The Clash, which was released in mid-December 1979 in the UK. However it was not released in the US until January 1980.
The arcade game that the Ultimart employee is playing during the shootout is the aptly titled classic Doom II. The game was never actually available in a commercial arcade cabinet and must have been constructed specially for the film.
After the Ultimart blows up, Martin gets in is car which is parked across the street and calls Debbie. The reflection on his car does not show the Ultimart across the street in flames.
When Martin closes his locker, Felix appears from somewhere down the hall. As the camera angle changes during the fight, we can see that Felix came from the alcove for a classroom door where Martin would have seen or heard him long before his appearance in the hall. The stairwell from which Felix SHOULD have come (from which Paul appears later) is significantly farther away from the locker - but that would have prevented the lightning-fast start of the fight.
When the bomb is rotating in the microwave, there are no sparks as would be present if metal were in a microwave when it is turned on.
All throughout the movie, the gunfire and muzzle flashes don't always match.
In the beginning of the movie, just before Martin sees Debi for the first time, sitting in her radio booth, he is driving in his car and looking to his right. He is wearing a black cap and sunglasses. The camera mounted in the open window of the car, is clearly visible in their reflection multiple times.
During the introducing shots of Grosse Pointe there is a shot of a boat, helicopter blades are visible at the very top of the screen.
When Martin first arrives at the Ultimart, a crew member with a boom microphone is reflected in the side of Martin's car.
When Martin and Grosser are discussing the 'Assassins Union' concept, the set lights are reflected in Martin's sunglasses after they discuss the hotel maid assassins.
When Martin is in the car with Mr. Newberry, palm trees are clearly visible in the background. There are no palm trees in Detroit.
In the street just before Martin sees Paul for the first time, there are mountains in the distance -- something definitely not found in Grosse Pointe.
At 1:12:26 Martin (John Cusack) and Paul (Jeremy Piven) run into Jenny Slater (Traci Dority) and Paul desperately tries to get her attention and when she remembers who he is she turns away seemingly in disgust, as if she wants nothing to do with him. About 6 min. later at 1:18:14 you see Paul, standing behind Martin & Debi (Minnie Driver), confidently grab Jenny by the arm and begin dancing with her. It as if both character's personalities do a 180.
When Martin calls Dr. Oatman the last time, he only dials seven numbers, yet he has flown the Detroit from elsewhere, and should have included a '1' and an area code.