Visible crew/equipment: On the runway, two shots after Castor Troy says, "This is a big gun. Fly the f*ing plane," there is a side shot of the Hummer. As the camera pans in closer, the reflection of a crew member on the filming vehicle, which moves beside the Hummer, becomes visible at the Hummer's back end, just before it cuts away. (00:10:25)
Continuity mistake: When Sean Archer is flying the helicopter to keep the plane from taking off, Castor Troy shoots the helicopter windshield three times, but there are four bullet marks.
Continuity mistake: After the second speedboat explodes, when Sean (as Castor Troy) pulls the speargun trigger, Castor (as Sean Archer) grabs the spear shaft before it can unload. As he grips the gun, the problem is that in some shots the slings are still loaded, but in other shots they are not. (02:09:00)
Revealing mistake: When Loomis comes upon Castor grieving over Pollux's body and says "Sir, why are you so upset? Its just Pollux Troy", you can see discoloration of a lump on his forehead which is makeup covering the blood pack where Castor will shoot him.
Continuity mistake: After being told that Pollux was released from prison, Sean Archer (as Castor Troy) shouts that he's Sean Archer, then grabs hold of the bars above him, with three bars between his hands. Next shot, as the guard's foot is on his hand, there are only two bars between his hands before he lets go. (00:58:50)
Revealing mistake: In the speedboat crash where the film is slowed down, there is a medium shot of Troy and Archer flying through the air. It is very obvious that neither are the true actors.
Continuity mistake: At Erehwon prison, during the cigarette incident, Sean (as Castor Troy) is knocked down, and while on the floor the side bar of his left magnetic boot breaks and the entire boot begins to fall off, but it's intact when he crawls to get the cigarette. (01:07:20)
Audio problem: When Karl is trying to push himself on Jamie, you can hear her screaming when the shot is outside the car, and the screaming continues when the shot's showing both of them, but she isn't screaming.
Continuity mistake: After leaving the Hummer, when Sean gets into the chopper the cabin interior changes in the following shots. This includes the windshield, hand grip, wall, etc.
Continuity mistake: During the speedboat chase, when Castor Troy (as Sean Archer) shoots up the white speedboat that Sean (as Castor Troy) drives, there are numerous bullet holes on its starboard side. Four shots later, when he hits the box on the rear seat, those bullet holes on the starboard side are gone. (02:04:15)
Continuity mistake: At the airport hangar, after Castor reloads, in the close-up Sean is kneeling beside a crate with a large black/white SCI air freight sticker. In the overhead shot, when Castor Troy aims the gun at Sean, the sticker is gone. Debris also disappears. (00:15:35)
Revealing mistake: In the scene when Castor Troy kills Agent Winters the sky is blue. There is only one cloud just above the fuselage. This cloud doesn't move, at least not that fast as it should as the jet is near take-off speed. The aircraft stands still when the actress falls out of the jet. In the ensuing shot, when the dummy falls out, the sky is overcast and the aircraft moving again.
Visible crew/equipment: When the red boat has hit the barrier and Castor Troy and Archer fly through the air, you can see a wire attached to the front of the boat just for a split second, then when it falls in the water and explodes.
Continuity mistake: After Sean looks up his son's file on the computer, and types "case closed" on Castor's file, he rubs his head with his right hand in the close-up, but next shot he is rubbing his head with his left hand. (00:21:20)
Continuity mistake: Eve and Troy visit Mikey's grave. When they first go out to the car, it's cloudy and raining, but they get to the cemetery and the sun is out and everything is completely dry.
Audio problem: When Travolta steals the speed boat there is a wide shot of him driving away and you can see Cage running towards him in the top right of the screen. You can hear Cage fire his gun but the muzzle flash doesn't appear until a second later after the gun is fired.
Continuity mistake: After the jet crashes into the hangar, Sean Archer runs up the back end of a police cruiser, whose three blue flashing lights are completely shattered. In the next shot facing him, when he reaches the car's roof, two of the blue lights have repaired themselves. (00:13:45)
Continuity mistake: When Castor Troy (as Sean Archer) turns off the video-cam and walks into the interrogation room with Pollux, the Coke has two straws, but next close-up there is only one straw as he turns off the microphone. (01:00:00)
Continuity mistake: When Castor (as Sean) grips the speargun, in the following close-ups, the sling ends (with its wishbones) differ - just note the rings around the rubber ends.
Revealing mistake: When Castor is pushed into the blowing heat from the plane engine by Sean and is thrown back, the man that lands against the grill is so obviously the stunt double; he has a different hair cut than Castor and really doesn't look anything like him.
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.