In the Firefly/Doctor Who crossover fanfic The Man with No Name, some of the characters are hunting Reavers. When they stop, thinking they've heard something, the Doctor keeps going anyway, walking smack right into one. Even the Doctor admits it was blindingly stupid.
Although, it does get a lampshade in chapter 63, when Moody mentions a drug called "Bahl's Stupefaction", the effects of which include causing the user to do extremely stupid things.
The cast of Hogwarts Exposed grab the Idiot Ball frequently, but special mention has to go to the scene in which Harry, Hermione and the entire Hogwarts faculty get outwitted by Crabbe and Goyle.
