Some of the encounters I design are intended to fool the players into taking the wrong action, with the intention of the players later realising their mistake and becoming amused by it.

This does rely on my understanding what my players enjoy.

— Merric Blackman (@MerricB) November 16, 2021

Consider a room in which are a group of menacing frost giants, who react to the door opening by looking around for the biggest rock to throw.

Not surprisingly, my party fireballed the room. But wait! It wasn't a room of frost giants! It was an illusion over a room of brown mold (which grows tremendously in the presence of fire). Nasty.

— Merric Blackman (@MerricB) November 16, 2021

This is an encounter that can be “solved” by waiting a little and not having a hair-trigger reaction of fireballing anything that seems dangerous. But it's also an encounter that very much amuses some players and DMs. (Some of which store it away to use themselves at some point).

— Merric Blackman (@MerricB) November 16, 2021