Continuity mistake: In the scene where Rico is getting ready to go to the end of school party, he puts on his black jacket and for a brief moment, we can see the collar is turned up at the back. During the following conversation with his parents he never adjusts his collar, yet when he turns his back, the collar is neatly in place. (00:12:35)
Continuity mistake: In the scene where Rico is first in boot camp and Dizz says she is being transferred to the unit, she gets ready to fight the drill sergeant. She drops her papers, her jacket, and her bag. It cuts away for a few seconds, then goes back to the drill sergeant and Dizz in fighting stance. The piece of paper is farther back than before, this can be overlooked because of wind, perhaps. But the jacket and bag are both gone. (00:25:55)
Other mistake: When the large dropships are releasing the smaller craft with the ground troops on board, the large ships are in orbit around planet B, but there seem to be two different types of physics at play - when the small ships are released, instead of using thrusters to push away from the large ship and begin descent into the atmosphere they are already using thrusters to slow the speed of the descent, even though they are still in orbit. (00:59:15)
Audio problem: When the troops are lined up on the fort's wall near the end preparing for the waves of bugs, Lt. Rasczak is walking at a fast pace behind them and shouts something before aiming his gun. Before he aims his gun, he is seen shouting the same thing as he walks past them, but there's no sound coming out of his mouth. (01:29:25)
Continuity mistake: At the scene where they get trapped at the fortress, you can see they are not shooting in the direction of the bugs. (01:31:25)
Continuity mistake: As the Roger Young is breaking up, the ship's captain is running from the bridge and falls down with the large door descending and striking her midriff. As it hits, she spews blood from her mouth against the side of the door in a arching/sweeping motion. When Carmen returns to help free her, the blood is not displayed in the same pattern, more concentrated to one area as opposed to swept. (01:44:10)
Other mistake: When escaping from the Roger Young, Carmen manages to pilot the escape pod through a debris field, re-enter the atmosphere, bounce off an outcrop of rock, through a rock wall, and bounce off another wall,all without wearing a seatbelt and managing to stay seated. (01:45:20 - 01:47:00)
Continuity mistake: Toward the end, Carmen Ibanez gets stabbed through the shoulder when she is brought before the big master alien thing. If you were to sustain such an injury, there would be so much swelling, not to mention just destroyed tissue, that you wouldn't be able to move that arm for weeks. Yet a few minutes later, she is seen hefting around a very large assault rifle, and seems to have no difficulty. I guess it's a pretty standard action movie "heroes can temporarily ignore injury in order to save the day" kind of oversight, but it just seems exaggerated in this case, considering how severe the shoulder trauma looked. (01:48:15 - 01:53:40)
Other mistake: Just after Rico and Ace rescue Carmen, the other guy has hold of the nuke, which explodes. Watch when the rocks collapse behind the three characters, you can see a nice square block lower from the ceiling, almost like a door being lowered. The crew could perhaps have made it look a little more realistic. (01:55:00)
Visible crew/equipment: In the scene towards the end, in the cavern containing the brain bug, there's a pyrotechnic set-piece of a massive quantity of ammo being used up. This was filmed from a number of angles, and in one shot, looking down across the cavern there is a camera man and an assistant visible in the mouth of one of the tunnels leading in. I don't know if this is corrected in the DVD release, but it was certainly there in the UK theatrical version.
Revealing mistake: When Rico jumps on to the back of the tanker alien and starts shooting at its back, you can see the outline on the back of the tanker that is going to be shot up before Rico starts shooting.