Moron Beam

The Moron Beam is a tool most often used by lazy writers to make otherwise-intelligent characters temporarily stupid for the sake of plot. The use of the Moron Beam can be identified by the screams of "NOOO! Why did you DO that?!" from fans who recognize that the character in question really should have known better. Fanfic, particularly badfic, is riddled with this sort of thing, but examples appear in canonical works, too.

If you must use the Moron Beam, it really ought to at least be lampshaded.

Canon Examples
Farscape: Surely the Peacekeepers built proximity sensors into their Prowler ships. Surely Aeryn should have noticed the pod coming down on top of her like that. Nope--they got her with the Moron Beam.

LotR movies: "I've got a great idea! Let's take the Hobbits to Osgiliath!" ... Poor Faramir.

Star Wars: "So this is how liberty dies -- with thunderous applause." Apparently the entire Senate got hit with it.