Quotes: Too Dumb to Fool

Chris: I was too busy watching a show about Superman where Clark Kent stands there watching his mom get shot on TV. The Blur, everybody: Noticeably Slower Than a Speeding Bulletô.
David: Whatís amazing about that is that even Lex Luthor assumes Clark doesnít suck that much. Iím jumping ahead a bit, but I just want to touch on this: I think Lex Luthorís overall plan here is actually kind of clever. Itís a good idea. Shooting someone Clark loves with Kryptonite bullets so that when he throws himself in front of them he gets messed up, thatís a solid Superman-busting plan. I dig that.
Chris: Except that Alexander, having never watched Smallville before, doesnít realize that this would require Clark to actually, you know, DO SOMETHING, thus dooming his plan to failure before it even starts.
Chris Sims and David Uzumeri on Smallville ("Beacon")

Major Bloodnok: "He may be a king's messenger. Let him in, but first, make him give the password, which is 'I don't know'."
Eccles: "Okay. 'I don't know'. That'll fox him. (Reaches the door). Erm, hello, my good man. Now then, what's the password?"
Seagoon: "I don't know."
Eccles: "Oh, he got it right first time.."
The Goon Show: "The Rent Collectors"

Glados: All right, paradox time. This...sentence...is...FALSE! Don'tthinkaboutit, don'tthinkaboutit...(Frankenturrets start shorting out and exploding)
Wheatley: Um..."True". I'll go "True". Neh, that was easy. I'll be honest, I might have heard that one before though. Sort of cheating.
Glados: It's a paradox! There IS no answer! Look! This place is gonna blow up if I don't get back in my body!
Wheatley: Uuh, "False". I'll go "False".

When you talk, sometimes I get confused. My ideas of what's right and wrong get mixed up. That's why I'm bringing this. As soon as I start thinking it's all right to steal from our employees, I'm going to start hitting you with the stick.
Helix, Freefall

It's like, even when we win, he wins.
Strong Bad, after he tricked Homestar to eat his own Stupid Uncle Egg's Good Time Diaper Pie... and he didn't mind it!

The little guy did something to your heads. I gave him the explodo because I am clever.
Tabby Smith of Nextwave, who escaped Forbush Man's mind control through this trope.

"One second he's dumb as an ox and the next he's sharp as a whip...that's why I hate him so much."
Izaya Orihara stating his opinion of Shizuo.

"Amazing. He'd actually have to be smarter to do something stupid"

Natasha Fatale: Well, why didn't goof gas affect moose?
Boris Badenov: Goof gas affects the brain...
Natasha: And?
Boris: No brain, no effect!