In AVP: Alien vs. Predator it is clearly stated that it takes place on October 10th 2004. Aliens vs. Predator: Requiem takes place immediately after the first film. But in the maternity wards two vital signs monitors show within a matter of seconds show two different dates. These are June 3rd 2006 and September 23rd 2005.
When the guy is firing the Glock from the helicopter, he empties it, the slide locks back, but sound effects are added for a revolver hammer falling on an empty cylinder. Glocks, and most other semi-autos, make no sound after the slide locks back and the trigger is squeezed.
At the beginning of the movie when the hunters find the downed space ship, one hunter uses the butt of his gun to tap the ship. While he is doing this, he points the barrel of his rifle at his kids head. This is not the safest way to use the gun, especially considering he is taking his young son hunting.
Ricky delivers pizza to Jesse's house, 1472. The number on the light post outside when he arrives is 1012. Later in the sewer to find his car keys, the markings show he's underneath the 1400 block of her street.
In the opening scene when you see Predator on the table before the pred-alien bursts out, the table is actually completely different to that of AVP: Alien vs. Predator. The markings in the first movie has Predator script reading vertical whereas in Aliens vs. Predator: Requiem they are just lights running horizontal.
The Greyhound bus is seen arriving in Gunnison at the beginning of the film, with the destination sign in the front window marked CRESTED BUTTE. Although Crested Butte is 28 miles north of Gunnison on CO-135, there is no Greyhound transportation of any kind to Crested Butte. Additionally, Greyhound offers no services on their fleet of buses to Gunnison itself, as these are contracted out to another bus company, Black Hills Stage Lines, Inc.
The Colorado National Guard are depicted as being armed with fully automatic M4A1 carbines. M4A1s are normally only issued to Special Operations troops. Standard Colorado National Guardsmen should have the basic M4 carbine, which has a three-round burst.
The Interceptor body armor scavenged from the slain National Guardsmen quite obviously do not contain the hard boron-carbide SAPI (Small Arms Protective Inserts) plates, which create a distinctive flat outline against the vest.
The goof items below may give away important plot points.
On the screen showing the purported "damage assessment" of the tactical nuclear weapon, in the lower part of the screen the bomb is designated as a "MK 83". In actuality the Mark 83 is not a nuclear weapon, it is a conventional weapon with a modest amount of high explosive inside. It would not be capable of the damage estimated on the readout. The correct designation of the nuclear weapon is a "B 83".
Before the helicopter lifts off, Kelly kills an alien by shooting through the windshield. Later, when the helicopter is in flight and attempting to escape the nuclear blast, the bullets holes are gone.
Leading up to and including the final battle atop the hospital, all of the action takes place during a heavy downpour. Later the same morning at dawn, when the survivors are found by the soldiers, the entire area is completely dry. While this is possible, based on the amount of time the helicopter was airborne, it is not very likely.
When the sheriff is leaving the group to go to the "airlift", and the other are deciding, Jessie's gun is visible against the light in the tank. The slide is shown to be pulled back, meaning it it out of ammo. However, she is yet to even fire her weapon, and she would still have bullets in the gun.
On Col. Stevens monitor displaying the bomber and delta force helicopter, the motifs used appear to be that of a Panavia Tornado (as opposed to the depicted F22 Raptor)and Westland Lynx (as opposed to the labeled Blackhawk). Both of these are European, despite being used on a display aboard a USAF E3 presumably built (and programmed) by US engineers.
Since Gunnison covers only 3 sq. mi., Colonel Stevens's map is showing a "pressure damage" radius of barely 1/4 mile. In reality a 200KT nuclear explosion would level all structures within at least one mile. The "heat damage" radius on Stevens's screen is even more understated.
Ricky gets stabbed through the shoulder by the pred-alien in the hospital and later uses his injured arm several times in a way that would be pretty much impossible after suffering such a major wound in that location, not just because of the incapacitating pain but because of the gaping hole he'd have where some of the necessary muscles would be.
When the F-22 Raptor arrives over the town and releases the "nuke" it is traveling at near supersonic speeds (to allow it to escape the blast radius of the weapon). However the bomb is seen to "free fall" at a near vertical angle, and only deploys its retarding fins moments before detonating. In reality, when deploying a weapon with retarding fins attached, the fins spring open immediately after clearing the bomb bay to maximize the available time for the aircraft to clear the lethal radius of the weapon. Also, the B-83 nuclear weapon does not employ retarding fins to slow its descent, it uses a Kevlar parachute.
When the Predalien implants the pregnant woman in the hospital, the anatomy does not work: The action is accompanied by a swallowing sound, suggesting the embryos are traveling down her esophagus, but the "location" of the bulges would place them in her trachea, which is at the front, with the esophagus behind. If that were the case, the actual sound would be a horrible, hollow tearing sound as the embryos forced their way through the cartilage that surrounds the windpipe. If the intent was to put them down the esophagus, they would in no way form such smooth, well-defined bulges on their way down, as the esophagus is hidden behind the trachea.
When the helicopter is riding the shock wave of the blast, the horizon indicator is shown spinning. This would only be possible if it were performing a barrel roll, which, not only is it not being shown as doing, but is impossible for a helicopter to perform.