Porter enters Mr. Fairfax's office with his semi-automatic pistol aimed at Fairfax. But he incorrectly holds the gun tilted 90 degrees to the left. Such a hold does not allow proper sight alignment, and could also jam a semi-auto pistol during firing. Nobody familiar with basic pistol operation would grip one in this manner.
When Porter is tied to the chair towards the end of the movie and he is being beaten by Fairfax's thugs there are several shots from behind that show Porter's shoes are on and that his legs are not tied. When Bronson comes and starts to question him these same shots are shown. Before they hit him with the hammer his shoes have been removed and legs tied with no one ever doing this to him.
When Porter and Val start driving down the alley towards the Chow's car they are initially in a 1970 Chevy Nova (round headlights, no front indicator lights, and no Chevy emblem on the grill) but just before crashing into the Chow's car it becomes a 1973 Chevy Nova (different grill, squared off lights, Chevy emblem on grill) when the two cars impact, the side shot is of the '70 Nova.
When Porter is hit by Pearl and her group of thugs' car he is shown shooting his revolver, usually a 6 shot, 13 times and possibly more off camera, but he is shown actually pulling the trigger and the hammer dropping 13 times in frame.
When Porter is in the back of the limo with his apartment exploding in the background, the door sill of the limo is visible showing that the door is open. But the next exterior shot shows all the doors closed.
When Porter is in the cab stand, the cop's badges are on their right side. When Porter and Stegman are talking outside you see the cop looking at them from the window, his badge moves to his left side.
When Val is talking to Porter while he is in bed with Pearl, Pearl reaches over to Val and her hair is covering the left side of her front. The next shot hand is still on Val but her hair is behind her and not covering her body.
At the meeting at the end of the line, Porter goes into the bathroom and takes a urinal between two members of The Outfit. The hit-man on the left pulls the gun out of his waistband. When Porter shoots him, the gun is back in his waistband.
During the opening sequence, Porter picks a man's pocket & steals his wallet. While practicing the face in the mirror, he's wearing the light gray jacket. But in the next shot, he reaches into the teller's window & she looks up at him in the black jacket he will buy later. When he buys & pawns the watches, light gray again. Then he buys the black jacket & has dinner.
When Porter is in the back of the limo, after his apartment explodes, he looks at the ignition and sees a Ford ignition switch with a Ford ignition key hanging in it. When the scene cuts to him driving away, he is driving a Cadillac, which is made by General Motors.
When Porter steals the detective's badge, the number on it ends in 88 (the left two digits are covered by Porter's thumb). When the internal affairs detective confronts the detective about his missing badge and shows it to him the number on it is 3887.
After Porter rolls out from under the Chow's car and shoots the driver, he stands up and then Pearl points an empty gun at Porter and pulls the trigger a few times. Porter then points his revolver at Pearl. When the camera cuts behind Porter looking at Pearl, Porter starts pulling the trigger and the hammer on the gun starts moving. When the camera quickly cuts to a side view Porter waits for a second then starts pulling the trigger.
When Porter Shoots Val in the head he uses a pillow to "muffle" the shot and it works, but the gun is a revolver and those can't be muffled at the muzzle as the sound can escape at the gap at the front of the cylinder.
While entertaining, an internal affairs investigation would never be conducted in the field as shown. The procedure would involve calling the detectives to the station so the interview could be conducted in a controlled environment and they would have the opportunity for legal counsel.