Troll level design means making levels that are meant to trick players by breaking assumptions.

Good troll design follows an important rule, but bad game design doesn't:

The task is strange the first time you face it, but you can see why afterward.

There are traps in level devil that are easy to miss. After experiencing them, players will be able to spot them and avoid them in future tries.

This turns failing into a chance to learn instead of just a punishment.