A "they almost got it, and are obviously intelligent" turns into "they froze up, but would have gotten it otherwise".

(To be clear: I've recommended hiring men that obviously froze up. I don't care how programmers perform in high stress environments. We're not trying to reboot the computer in Jurassic Park, FFS.)

That would be stressful, but not only that. Stress may come from pretty “calm” sides, like the lack of task explanation or illogical authoritative statements, indecision due to vague constraints, unreasonable time pressure, monotonous grind, working hard weeks right into the trash without any explanation (to name a few just from the top of my personal experience). Stress is something that bothers you so much for months or years but you can do nothing about it.