At the beginning of Propeller Knight's stage, there are some enemies, which look like they could be platforms, over a bottomless pit. On my first attempt at this level, I jumped on the first one, thinking it was a platform, and got hit and fell into a pit, leading to an early death that I don't think was fair.

This problem could have been solved by putting some ground on the bottom. This way, a player who tries to jump on the first enemy would take damage, but land on solid ground, allowing her to get used to the mechanic.

The Clock Tower handles this situation better; on the first screen, there are conveyor belts (so the player can learn how they work), but no pits (so the player isn't punished for not knowing how they work); the pits come later, when the player is already familiar with the mechanic.

Of note, the original Mega Man had a design flaw of this type; see the beginning of Guts Man's stage from the point of view of somebody who hasn't played it before.