Gail's earrings, although supposedly the same pair through the entire movie, are two different pairs. One pair is a clip-on which she wears backwards, with the ear pad in the front instead of the back. These are worn high on her earlobes so you can see her pierced hole beneath them. Later, after she is kidnapped by Manute, the earrings are pierced with fishhook posts. After the girls take down the mob, she is back to wearing the reverse clip-on pair. The earrings were very long and dangly and would have been a potential hazard in an action scene. To avoid tearing the actress's lobes the two pairs were used.
When Marv steals the Police Car and ditches it in the water, he gets out of the car while underwater and swims away and comes up through a drainpipe. Once he emerges from the drainpipe, his hair and clothing are dry.
In the distance shot of Marv and Wendy driving to Kevin's Farm, we see Marv smoking a cigarette. When the camera cuts to a close-up of Marv and Wendy's faces, the cigarette is instantly out of Marv's mouth and in his hand.
When Hartigan is being beaten before going to jail, there is a close-up of his face. There is a five-inch long, one-inch wide rip in his right cheek. Even though he has scars already on his forehead, this damage just disappears afterward.
When Wendy first tries to hit Marv outside the church, his back is to the door of his Mercedes. But when she actually hits him he is parallel to the Mercedes; in his original stance, Wendy would have run into the car.
When Hartigan sits in prison, he says he "refused to plead innocent." This is not correct terminology. An accused person can generally plead "guilty" or "not guilty." There are other possible pleas, but this is irrelevant and none of them are "innocent" for various reasons.
It cannot be a character error because Hartigan is a highly experience policeman with personal interactions surrounding his own court case to boot.
As soon as Hartigan realizes he was setup to find Nancy, he tries to leave unnoticed so Junior doesn't know who she is.But Hartigan didn't know what she looked like, he just asked for Nancy. But Junior knew her name was Nancy too, so he could've just asked for Nancy even if Nancy never spotted Hartigan.
Even regular patrol cops knew not to enter Old Down with their sirens on and feared the area to the extent that they disengaged pursuits, turned off their sirens, and left. Jackie Boy is supposed to be an experienced detective and a "hero cop" to boot, but he goes to Old Town and foolishly threatens a hooker with a handgun, getting himself killed. It's something everyone except him and his goons easily foresaw, but the detective inexplicably couldn't figure it out.
Flopped shot: A brief shot of Marv after he has been scratched by Kevin is flopped, the scratches run from bottom right to top left, instead of bottom left to top right as they do when he received them.
When Dwight is hallucinating while driving to the pits, specifically during Jackie Boy's line, "Would you look at that," when Jackie Boy bends his head back, you can clearly see the holes through which fake smoke comes out his neck when he smokes.
The human bone Marv digs up at the farm is clearly a part of model skeleton rather than a product natural decay. Several holes are visible in the joint at the top where metal pin attaching it to the model was removed.
The goof items below may give away important plot points.
When Hartigan was shot in the back by Bob, we could see the location of everybody on the pier. Bob is in the back, young Nancy is sitting, Hartigan, and then Junior is on the floor. Hartigan staggers a little and yet in the next shot he's at the pier's edge which is 6 feet from where he should be.
The Irish assassin shoots Dwight with a Steyr AUG, which is chambered in a .223. When he is examining the badge that stopped the bullet, the slug that is stuck in it is much larger than a .223 would be.
Stuka makes a point of not moving or reacting physically in any way when Miho shoots him with an arrow. So when we pull back to long shot, it's clear from where he's standing the arrow must've come in through the brickwork, not the window.