When Mark is aiming the pistol towards the nurse (Cecily) at one point he holds it with two hands, but when the camera makes a fast angle switch he's holding it with only one hand.
Outie Dylan's handwritten resignation letter is written differently from the one Innie Dylan reads. For example, "Point two:" is all on one row for innie Dylan's letter, but on outie Dylan's, it's separated into two different rows.
When Drummond is seen loading the captive-bolt pistol that is to be used to kill the goat, the round that he loads is clearly a normal round (which is a case with bullet pressed in), not a blank. Bolt pistols use blanks - a normal round would not work and would be dangerous or deadly to the operator.
The directions to the elevator start from O&D. However, once Mark looks at the directions for the first time and starts running down the hallway, he keeps looking at the directions even though they're useless until he's reached O&D.