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.
Revealing mistake: When they are learning how to throw knives, the guy gets one thrown through his hand. If you watch this in slow motion, blood starts coming out of his hand before the knife hits.
Continuity mistake: In the first battle with the Roughnecks, Rico, Diz, Ace, and the other guy are confronted by a bug and they shoot at it. If you look at where they are aiming they are shooting above it, but the bullets hit it in the middle.
Visible crew/equipment: When they capture the brain bug, just before Carl says "It was a man called Zim [who captured the bug]", you can see the shadow of the camera on his shoulder. (It was the DVD version, not the cinema).
Factual error: Rico lives in Buenos Aires, which has no hills within a couple of hundred miles. The city shown in the film has hills.
Continuity mistake: Regarding Lt. Rasczak - in the scene where he is teaching his students, you can see his left arm where it was previously amputated, and how close it is to the elbow joint. But later in the movie, when he is introduced to Diz, Rico, and Ace in the Roughnecks, you can see that his arm is much longer than where the amputation from the previous scene would have been, even with the "implant" hand that he now has.
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.
Continuity mistake: In the first mission with the roughnecks Rico defeats the acid spraying tanker bug but when he's thrown from it and lands on his back his helmet chin strap breaks free and lands behind him. Next scene it's back on.
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.
Factual error: When Rico is whipped, the guy with the whip is shown about 15 feet in the background. The lashes leave huge 18-inch stripes on Rico's back, implying that the whip is about 17 feet long. But the maximum effective striking length of a whip is only 12 feet, at most.
Other mistake: During the live fire assault training, Pvt. Breckenridge has trouble with his helmet, and Squad Leader Rico removes the helmet to inspect it. A third private stumbles and falls nearby, and as she hits the ground her weapon discharges once. The camera immediately cuts to another angle, and the woman is still falling, hitting the ground again. Her weapon discharges several automatic rounds this time, swinging wildly from left to right. She is thus firing from ground level, pointing up at Breckenridge and Rico, who are kneeling only about 8 feet away. The automatic gunfire at that range should have struck both Rico and Breckenridge. As an aside, Breckenridge is shot in the head, and Rico is consequently disciplined for allowing Breckenridge to remove his helmet and getting him killed. However, Breckenridge was shot from below, in his right eye, blowing off the top of his head on the left side. Even if he was wearing it, the helmet would never have saved Breckenridge from a gunshot to his eye.
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.
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.
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.
Revealing mistake: During the last play of the football game, Dizzy throws Rico the ball and he does an impressive flying flip over several players. Right before the flip, it's extremely obvious that Rico runs up an off camera ramp/stairway in order to perform this flip.
Revealing mistake: Just before Dizzy gets skewered by a warrior bug, you can see breakaway panels on her body armor at the points where the CGI bug impales her, to cover the squibs used to make the holes. The shot is already shown in slow-mo so it's easy to spot.
Continuity mistake: In the scene when they are doing the live fire training course and the big guy gets shot in the head the bullets rip about 1/4 of his head off, but in the aerial view you can see his head is completely intact.