Continuity mistake: During the cemetery scene where they visit the son's grave, you see a wide shot of them walking between the stones. There are leaves on the trees and neatly trimmed grass. When we see a close up of the stone and the plaque in the ground, a hand reaches down to brush the leaves off so they can read it, and there are dead leaves all around - but there were no leaves on the ground in the wide shot.
Continuity mistake: When Jaime is screaming, Castor Troy (as Sean Archer) walks out to the front yard and the back tires/end of Karl's car is on the sidewalk, but when Karl is dragged out, the car is farther up the driveway. (01:24:50)
Visible crew/equipment: When Buzz and Wanda stand in the outer room, in the first shot, the camera pans around and faces the agents' TV, showing the video feed of Pollux in the interrogation room singing about his tiramisu. On the agents' TV screen, the reflection of the crew's rather large black square screen is visible (on a stand) in front of the windows (to block window light and view of the crew/equipment) and just as the camera faces the TV head-on, a crouching crew member begins to stand (seen at the left side of their TV screen). (00:59:05)
Continuity mistake: During the prison breakout, when Dubov falls over the railing after being shot, in the first overhead shot he falls backwards, but next shot he falls forward (chest down). (01:11:40)
Audio problem: When Castor wakes up from his coma, he reaches up to his face and pulls out the nose plugs that fed him oxygen. Strangely enough, his heart monitor stops beeping when the plug is removed.
Continuity mistake: When Troy (as Travolta) is about to defuse the bomb, watch the timer, it goes down too slowly for the time elapsed. It shows 1:14 and 28 seconds later it only shows 1:04, not 0:46 as it should. (00:58:45)
Continuity mistake: When Castor arrives at the airport, behind him there is a parked car in front of the building which on it reads "Welcome to Southern California International Airport", that disappears and reappears.
Continuity mistake: After Sean types "case closed" on Castor's file, Dr. Miller walks into Sean's office and holds out Pollux's Zip Disk for Sean to take. In the next close-up, the Zip Disk has flipped over and faces the opposite way.
Visible crew/equipment: When Archer gets into the chopper and takes off, next shot is of the chopper flying towards the camera and it is not Archer who is flying it - the chopper pilot wears head gear and a mic. (00:11:40)
Continuity mistake: After the incident with Karl, when Castor (as Sean Archer) and Jamie are in the living room, he opens the cigarette case and offers her one. The way he holds the case noticeably differs between shots.
Revealing mistake: At Erehwon, Sean (as Castor Troy) is given steel magnetic boots to wear, for the prison's important security feature. There is a close-up of these heavy duty boots during his 'exercise'. Then, when he is taken to the clinic there is another close-up, and there are a number of changes to the boots style. More importantly, one of the side "bolts" has fallen off, revealing that it was merely glued on the prop boots.
Revealing mistake: Just as the red speedboat heads toward shore, before it crashes, the white and red truck cranes are on the dock and the white crane arm is angled backwards over the dock. In the shot facing the speedboat, as it explodes, that crane arm is now angled the opposite way, over the water, since it was used in the stunt.
Factual error: In the scene in the hall where there are is a circle of mirrors where Castor and Sean engage in a shootout, there is a mistake as the FBI agent fires at Castor. The FBI agent is holding an M16 or M4 assault rifle with an M203 40mm grenade launcher. He fires two consecutive shots within a second of each other. This is impossible as the M203 attachment only holds 1 shell at a time and would take at least 5 seconds to grab another round and reload and fire again.
Continuity mistake: When Castor (as Sean Archer) grabs the speargun shaft before it can unload, he takes a piece of glass and begins to cut his face. The broken glass has a small circular piece missing from the edge, but from a different camera angle that small piece is more intact than the previous shot. (02:09:15)
Continuity mistake: When Castor Troy (as Sean Archer) says, "Every time you'll look in the mirror, you'll see my face," he proceeds to cut his face. The first cut he makes is straight down the side of his right cheek (viewer's left) and later that cut is curved and closer to his nose. This may appear to be a mistake at first glance; however, Castor's facial expression is different in the two images. When he made the cut, the cheeks were pulled away from the nose; in the second photo, the muscles are relaxed, moving the cut closer to the nose. (02:09:15)
Continuity mistake: At the hangar, when Castor Troy is blown backwards in the turbo tunnel he hits the wire gate, and the metal bar, about 3-4 ft long, breaks off and begins to fall with him. Next shot, he lands on the floor, but the long bar does not. It just disappears. (00:17:45)
Visible crew/equipment: When Cage throws Pollux through the ceiling window, the support wire is seen in front of Cage as he's hanging from the edge.
Revealing mistake: In the hall of mirrors Travolta runs behind the mirrors in order to get the drop on Cage. Cage starts shooting, but when he stops and lowers his arm the muzzle flash (and possibly the gunfire as well) from his gun continues as if he's still firing.
Continuity mistake: After the surgery, when Sean (as Castor Troy) breaks the center mirror of the tri-fold mirror, what remains are the two side mirrors and white backing within the center frame. When Tito grabs him as he starts cursing, in these close-ups the reflections seen are as if all three sides are still intact. (00:37:15)
Continuity mistake: During the runway pursuit, there is a shot from behind Castor Troy as he fires on Sean's chopper, and there are four bullet holes on the windshield. In Sean's close-up the four bullet holes change position.
Chosen answer: Although they were on the same side, Troy is currently posing as Archer, which means he would have to do everything that the FBI would expect Archer to be doing. The whole point of the raid was to take out Archer, as well as Troy's gang. He would have rather risked killing part of his own gang than risk exposing his identity to anyone else.
Casual Person
That doesn't really make sense. In the scene, he goes out of his way to shoot him and smiles while doing so, carefully and slowly. Was not a collateral damage situation. The question is why he deliberately goes out of his way to kill him.