When they first arrive at the fire where they rescue the fat guy, you can see that the firefighters are just aiming their hoses randomly, not at the fire. One is even spraying the side of the building where no fire is even visible.
During the scene where the other firefighters refuse to play a game of basketball with Chuck, as the firefighters are shown walking away, the character Nootzie's gray shirt shows a very light sweat stain. When they cut to Chuck and then back to the firefighters Nootzie's shirt has a darker, more pronounced sweat stain.
In the rooftop scene with the guy stuck in the chimney, a large cable hanging from a telephone can be seen directly in front of the Empire State Building. It disappears when the building is seen between the his legs. The director had it digitally removed from the shot, but didn't want to pay to have it removed from the other ones.
As Duncan breaks into song during the shower scene and Chuck and Larry provide back-up vocals, Larry turns around to see him sing. In one shot from one angle, Larry is holding a bottle of shampoo. In a different shot from another angle Larry is holding a tube of body wash.
During the scene when Chuck is sitting on the bed talking to Alex, his "best friends 4-ever" bracelet changes positions on his hand as he touches her face. One shot it is nearest his wrist and the next shot it is moved down his arm further. The letters on the bracelet change positions throughout the scenes. The bracelet could not had slid down his arm because the material the bracelet is made out of is tight on his arm.
The scene where Chuck and Larry extinguish a fire at a diner caused by an employee smoking marijuana, Chuck uses a fire extinguisher to spray that employee in the face. The extinguisher has a cone shaped nozzle which is indicative of a carbon dioxide extinguisher. This type does not leave any residue like in this scene which leaves a bunch of powder.
Larry resorts to naming Chuck as his life insurance beneficiary because Larry is concerned that the dangerous nature of his job may result in his untimely death and leave his kids alone. So he counts on Chuck to take care of the kids in case he dies. The problem is that Chuck has the same dangerous occupation as Larry, in the same firehouse, on the same shift and engine.
When Chuck & Larry are joking about eating the rat and the floor falls through, the entire building and everything in it is burnt, but the papers that fall to the first floor are all clean and unburnt.
The goof item below may give away important plot points.
In the scene in the jail cell, when Larry and Chuck are looking out the window at the gay protesters, Larry is not wearing his jacket. However, when the camera angle changes to show him talking to the man, he is seen wearing one.
During the trial scene while the firemen are giving reasons why they should be arrested also, Kevin McDonough is already standing up. When its his turn the to speak the camera catches him rising up to speak.