When Winston shoots the well-dressed black man in the hotel room the gun he is using does not have a suppressor on it. Yet the weapon is making the sound of a suppressed pistol. When he shoots the final shot for the kill you see this perfectly.
When Jerry initially retrieves his revolver from his luggage before going into the bar to find Beck, he opens the cylinder and a couple of cartridges fall out. In the process of reloading, he spins the cylinder and several clicks are audible. An open cylinder on this type of firearm would spin freely and would not click. (The clicks might be heard as part of the lockup mechanism if the cylinder was spun while closed.)
In the bar where he meets Beck, Jerry orders tequila and takes a sip from the shot glass, leaving it a bit over half full. A moment later, just before the bartender drinks it, the shot glass is completely full.
When Jerry confronts the men who stole his rental car, one of the men is wearing Jerry's purple sunglasses. Jerry gets irritated and takes them from him. However, before Jerry takes them back, there is a brief shot showing Jerry already wearing them.
When Jerry is offered an American car at the Mexican car rental agency he expresses disappointment and is offered an "El Camino" by the rental agent. However, in the next scene we see that several workers draw to see who gets Jerry's business, insofar as to who will allow Jerry to rent their car, but only one of the cars is an El Camino - which in the previous scene was already rented to Jerry.
When approaching the tractor/trailer around the 1.5 hour mark, for some reason the passenger door on the tractor is open, after passing the tractor the door can be seen to be closed in the rear view shot from the El Camino.
When Jerry takes the pistol back from the man who stole the el Camino, you can see the rag the pistol's wrapped in fall to the ground when he puts the pistol in his pants pocket. The next cut shows the rag hanging from Jerry's pocket.
When Leroy and Sam are in the airport and talking to Jerry on the phone, the time on Leroy's watch jumps from 8:35 to 8:45 between shots. And then when they are sitting outside waiting for Jerry, the time on Leroy's watch is 2:30.
After Leroy takes Samantha from the mall, her car does not have a rear license plate, a few minutes later when he pulls over because she is feeling ill it has a rear license plate. Additionally, when they are speeding down the freeway after leaving the mall the car has acquired a front license plate.
When Jerry swerves into oncoming traffic at the airport there are fresh tire tracks following the same path the car is taking towards the telephone pole, including the swerve at the last moment before the car would have hit the pole.
When Sam enters the plane you can see the last two passengers board and then the stewardess walks away. When the shot cuts back to Jerry, in the reflection of the glass you can see the last two passengers board again, and then the stewardess walks away, again.
When Leroy and Samantha leave the mall they drive on and on through the desert and end up in downtown Las Vegas at the Plaza Hotel. Maybe they just wanted to do a lot of driving, but in actuality the mall is on Las Vegas Blvd about 15 minutes driving on the strip to get up to the Plaza Hotel.
The film opens with a car crash at an intersection clearly labeled "Laurel Terrace", but when Samantha explains the story later in the film she says that it was by "Laurel Canyon and Ventura". However, she wasn't present at the accident, which was five years earlier, and it's understandable that she'd misremember it as the major street Laurel Canyon instead of the more obscure Laurel Terrace. Further, even if she knew it was Laurel Terrace, she may have used the nearest major street for clarity.
After saving Samantha's life, Winston puts the latex gloves he used into the trash, enabling police to lift his fingerprints off them.
Winston does take his gloves off, however he does not place them into the trash can. Winston only takes them off in front of the trash can.
The first time Jerry tucks his revolver beneath the dash of the El Camino (while Beck is relieving himself outside the bar), the orientation of the gun would indicate that the right side of the firearm is being shown to the audience; however, the cylinder release is clearly visible, indicating that this is actually the left side of the gun and the shot has been reversed. (Also note that he uses his right hand to stow the revolver but the orientation of the hand, especially the thumb, would indicate a left hand. This is further evidence of a flopped shot.)
The goof item below may give away important plot points.
About two thirds into the movie, we see Sam and Winston (false Leroy) have breakfast at a diner after having partied in the hotel room the night before. Winston shares with Samantha, how his new lover, whom they picked up on the way, found out about what he does for a living by finding the gun that Winston hid before they had started to party. The night before, we see Winston slide the gun under the cushion of an armchair. However, when he tells Samantha that story, he talks about having hid the gun between towels in the bathroom.