When Bryan Metro gets out of the bath tub and puts a towel around his waist, he is completely dry.
When Graham wakes up hearing shots outside the hotel, Martin has briefs on in the introductory long shot, but is completely naked in the following close-up.
In one of the earliest scenes the song "Dance Hall Days" by Wang Chung is playing in the background. "Dance Hall Days" was not released until 1984, a year after the film takes place.
At the start of the movie there is a segment of sped-up-traffic, in one view of an intersection a VW New Beetle is clearly visible stopping at the traffic lights. The VW New Beetle was introduced in 1998, long after the events of 1983.
The first traffic scene shown there is a C-4 generation Corvette (came out in 1984 there was no 1983 model) and a first generation Ford Taurus (came out in 1986).
At the wake when the guys are sat on the balcony they're wearing Wayfarer sunglasses with Rayban written on the lenses. This was not done in the '80s.
When Peter (Mickey Rourke) shows Jack (Brad Renfro) the child in the van, you can easily see a crew member through the front windshield looking in. He tries to get out of the way, but much too late.