Why Didn't I Think of That?

Oh great, Alice is making one of her "suggestions" again, and Bob is just getting ready to deploy the mandatory response when Alice suddenly begins to make sense. "Hang on!" Bob says. "Thats a brilliant idea... why didn't I think of that?

This is when a character suggests something to another character that they wish they'd thought up, or are even surprised they didn't think up first. Despite the opening this doesn't have to be a previously dim or spacey character surprising a smart one (though that version may overlap with Dumbass Has a Point); simply suggesting something that in hindsight seems obvious can catch any character off guard. Compare Glad I Thought of It, where the character not only wishes they'd come up with the idea but tries to pretend they did come up with the idea.

There's a tonne of examples on this IMDb search result... unfortunately most aren't in context, so if anyone is familiar with any of the films and series here, please add them to the examples with context.

Comedy
"Oscar Wilde: I wish I'd said that. James Abbott McNeill Whistler: You will, Oscar, you will."
 * Oscar Wilde

"Berle: [quoting back a punchline that was just said]. I wish I'd said that. Other person: You will!"
 * Milton Berle, who was memetically known for stealing other people's jokes, would often use this on himself (and obviously, the bit itself was stolen from Whistler, above).

Live Action TV
"That was an easy joke. I could've thought of it. Why didn't I think of it?"
 * Friends:

Web Comics

 * Exterminatus Now has this. Indeed, Harold. It's strange.

Western Animation
"Dick Dastardly: Wow! What a keen booby trap! I wish I'd thought of it!"
 * From Wacky Races, after the Mean Machine is crushed by a hillbilly's boulder trap:

"Fry: Leela, you have to get me out of here. It's horrible! Eating scraps, letting my waste drop where I stand like an animal in the zoo. Leela: Animals go on the corner. Fry: The corner! Why didn't I think of that?"
 * Futurama, "A Biclops Built For Two"; Fry is in a dungeon:

Real Life

 * Thomas Huxley's response to Charles Darwin's theories of natural selection was "How extremely stupid not to have thought of that!"