Plot hole: When Mark slips and falls in the staff lounge, he puts both hands on the table to catch himself - but we can see him (the actor) quite deliberately lower his head to one hand - this is supposedly where he cuts himself, but he actually hits his hand and not the table. This would not produce the deep cut he ended up with. Also, the head makes contact on the left of his forehead and he ends up with a cut on the right. And when we actually see the cut, it is vertical and logic tells us that it would have been impossible for him to have a vertical cut from the angle he fell.
Continuity mistake: Nick the dog is standing in the sink and Mark and Doug have got him just about completely covered in soap. The dog jumps out and runs out of the room - by the time he reaches the hall, only a couple of seconds later, the soap has almost completely disappeared.
Revealing mistake: When Mark goes to treat his cut, he leaves the lounge with a couple of tissues held to his head. He gets some supplies and puts a supposedly fresh piece of gauze to his forehead - however the gauze already has some "blood" on it - a feature the camera obviously wasn't supposed to catch. When Jeanie comes over to check it out, he pulls the gauze away from himself and - voila - there's the bloodstain.