The BFG cells as picked up are square, yet are round when put into the weapon.
Mars's gravity is only 45% as strong as Earth's gravity, yet the Marine when outside of com-pressurized buildings walking on Mars's surface, is still walking on the ground as if there is no difference in strength of gravity
Weapons eject shells from the left side of the weapon. Weapons are usually designed to eject shells from the right side of the weapon and away from the firer's face.
If the hull of the base gets compromised, there s no blast of outgoing air from either the hole or the door leading back to a pressurized area.
After turning on the air filtration system at Recycling Sector 2, the marine still coughs, when passing by a vent, as if there still would be toxic gases spreading out.
The word 'scandal' is misspelled 'scandle' in a newscast.
Spelling mistakes in in-game text.
The biometric information on each PDA is identical in spite of belonging to different people.
Some zombies are headless, yet when they are gibbed the gibs contain a brain.
The suits worn by all characters are short sleeved, which would make them incapable of protecting the operator in a low-oxygen atmosphere.
In the video disk video for the Molecular Fuel Storage Compactor, the text only says 'Union Aerospace' instead of 'Union Aerospace Corporation'.
The ammunition storage cabinet near the Molecular Fuel Storage Compressor is behind the Compressor itself, which would hardly make sense since it would mean climbing over the Compressor in order to get to it.
Many levels appear to overlap each other. A good example of this is the "Central Processing" level, where many areas of the map appear to overlap where areas in the previous level, "Delta Complex", would have been.
In Dr. Raleigh's audio log about a suicide, he explains that the patient was saying things in English and other languages, but in the next sentence only mentions one other language and not more than one.
In the Delta Labs, Dr. Michaels mentions one storage cabinet in the next room, but there are two.