Homeless for the Holidays - S3-E10
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.
Answer: A man named Paul Sobriki comes into the ER in the season 6 episode 13 " All in the Family." He is complaining of headaches, Carter and Lucy decide to rule out meningitis and do a lumber puncture. Paul struggles during the procedure and believes that both doctors are stabbing him. Lucy, who is interested in Psych thinks something is wrong with their patient mentally, but Carter is dismissive of her. She gets cross with his attitude and she calls the psych department anyway to have a look at him. Later on and near the closing 20 minutes or so of the episode, all the staff start playing loud music and get a cake which appears to be blue lol, together to party as it's Valentines Day, Carter returns from somewhere and asks where Lucy is. He goes to check one of the rooms for her and Paul Sobriki comes up behind him in the dark and stabs him twice in the lower back, he collapses and as he falls unconscious he sees Lucy on the ground also. In the next episode, Be Still My Heart (ep 14), Dr Weaver discovers them and all of the staff work to try and save them, whilst Carter survives. Lucy's wounds are fatal and suffers a pulmonary embolism as Corday and Romano try to save her. Carter is grief stricken and feels guilty about the way he treated her, he becomes addicted to painkillers and Benton, Greene all help him and book him into rehab.