Sometimes on the first level, after you follow Groves' finger, Groves won't leave the room and you may be stuck in the restraints, unless the Scientist accompanying Groves lets you out.
During the level Jungle when you get to the room where 6 attempts to kill a Marine (the second building in the compound), if you go to the door where the Marine is facing a sentry gun just left by the gate will kill you. Before and after you are given your orders, the turret is shut down This is probably because the door isn't really meant to be opened by the player. So when the door opens the game spawns the Marine with the shotgun and it immediately kills you. Technically this cannot be called a glitch.
During the final boss fight with Wolf, sometimes the music will not play or the health bar will be missing.
Sometimes attacks will not connect showing you hitting that character but they took no damage or were stunned.
When trying a the finishing move when you stab them in their eyes you will still see some of its outer teeth.
When preforming a stealth kill on a predator (and succeeding) their mask will still be attached to them
Your fingers and tail will phase through walls.
During the Jungle mission, if you kill the marine that a Facehugger is scripted to attack, then said creature will attach to one of the other two marines but because its not scripted, they will not be attached properly i.e side ways on the face. Also scripted events where Facehuggers are already on host's faces such as the scientist in the chair or the marine on the floor during the first encounter with the parasites, the Facehuggers will not be pulsating like those on the harvested's faces.
In the jungle level and beyond in the campaign, your shadow will show the shadow of a Praetorian, despite other cut scenes
In the first level (after the tutorial) your elder may not speak making you incapable of leaving the beginning.
Aliens may occasionally stealth kill you but this is very rare.
When searching for the ancient wristbracer in the labs the ground may dissapear and you will fall to your death.
During the Predalien boss fight it will run after you and may not give chase as it is busy running into a wall which allows the level to be beaten even quicker. However when the player has gotten to the point where it goes to bring down the arena, it will leave.
Sometimes if thrown into the lava it will stay under and not bring down the arena.
After the Predalien dies the game may think you died and force you to start over.
You can kill the Abomination during the final time it tries to bring down the temple (easiest with plasma caster), the Abomination will then die and you will still see the temple falling apart, but then it will start the next cutscene. Technically this cannot be called a glitch.