Inaccessible green armor vest in Doom/Ultimate Doom level
E1M2, Nuclear Plant:

(Note: For a YouTube video covering this topic click here.)

In this level a player can find a green armor vest sitting
on a shelf across the room from the level's entry point.
Some players may attempt to pick it up, but fail to do so.

In Classic Doom games, armor vests come in 2 types: Green,
which provides 100% armor (and will absorb one-third of
incoming damage), and blue, providing 200% (and absorbing
half damage).

You can only take a green vest if your armor is currently
below 100%, and only take a blue vest if armor is below
200%.  If you got a vest (especially the blue one) on the
previous level, then there's a chance your armor is already
100% or more when entering this level.