Oh man, this brings back memories of the time me and my friends lost their entire party of D&D characters to a single monster that is only worth 35 XP.
I can't remember what it was called, but basically it was a tiny fungus with one eye that lurked in a corner somewhere. When it saw a character that had any spells ready, it would instantly steal all of the spells from the character and then cast the spells (randomly) at the party.
A whole party of 7th-level characters, wandering through a ruined church in the swamps...massacred by a mushroom. We never even saw what was killing us, we just started dying due to spells being cast at us out of nowhere.