I believe this one is a very old story, possibly even from the "Golden Age" of science fiction.
The premise is this: a new crewman, who doesn't seem to be cut out for life aboard a spaceship, continually makes mistakes and/or goofs up from lack of attentiveness to duty. The rule in the story is that a crewman who makes three serious errors on a mission is kicked out of the service. I can remember two of the three screw-ups.
The first one I remember deals with the airlock on the ship. The ship is on the surface on an alien world, and the environment is a toxic one. Decontamination before entering the ship is therefore essential. When the captain and another crewman return from collecting samples, the realize the decontamination tanks are empty, because the rookie forgot to refill them with fluid. Disaster is averted due to some quick thinking by another crewman, but the kid picks up a strike against him.
The other one I can remember, and IIRC this was his third strike/demerit, was the kid forgot to properly seal a specimen case or something like that. The samples react badly to an oxygen atmosphere and I think catch on fire, nearly asphyxiating a sleeping crew. The kid is devastated to know he's going to be booted out when they get home, because of his third mistake.
The story ends with the captain taking the disgraced young crewman with him on a final sample collection trip. Almost immediately, the kid cries out, and dies a violent death the moment they exit the airlock. The captain discovers that one of the attaching screws on the crewman's helmet hadn't sealed properly and, in a freak accident, the poisonous atmosphere of the planet had gotten inside the other man's suit.
That's all I remember. I couldn't find anything in search, so hopefully this is not a duplicate of a previous question (apologies if it is).