An item which harms the player who picks it up or uses it instead of helping them. Differs from Power Up Letdown in that the harm it causes is direct and intentional, rather than from a design flaw.
So, if it's harmful, why would anyone pick it up or use it? Players don't mean to, but poison mushrooms have three main ways of getting around that:
Sometimes they just wait in strategic locations [...] [tvtropes.org]