Making Themselves at Home
The Movie: The crew of Prometheus find the alien moon that the star map so specifically points out, no problem. As they are entering the atmosphere, Captain Janek performs a safety scan of their surroundings. They land next to a weird space temple and walk right on in. Finding the CO2 levels are nice and comfy, they say "see you later, helmets!"
Real Life: Why didn't Janek perform the scan before he was actually in the atmosphere? What good is a readout that tells you "actually it's a hot acid bath in here" when you're already in said acid bath? Also, if you ever find yourself in an abandoned space temple on some godforsaken moon, it's probs not a great idea to remove your helmet. The CO2 levels are fine? Cool! Never mind whatever else is in the air around them... airborne bacteria, mold, toxins, freaky alien spiders, literally anything. This isn't even Science 101, it's Self Preservation 101.