A Poison Mushroom is an item in the Mario series that, if obtained, will kill your player. It first appeared in Super Mario Bros.: The Lost Levels in Japan, where, if obtained, would kill the player. It looks similar to a regular Super Mushroom, and thus could trick the player into collecting it. Apparently this is one of the many reasons why the game wasn't released in America or Europe - because it proved to be a challenge to many gamers.
The Poison Mushroom also appears in Super Smash Bros. Brawl. The red on it is a little bit darker than a Super Mushrooms and its eyes look evil. You will shrink if you touch one.