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FuzzyWulfe
Since: Nov, 2010
thunderlog
Since: Dec, 1969
Chariset
Since: Jan, 2001
2011-09-17 13:58:55
It seems to come from the same comedy family as Conjunction Interruption (which can also end with characters saying "But—!" and then realizing they don't have anything else to add.)

Pretty sure this only happens in western animation. The character goes to say something, sometimes with his finger pointed up to denote knowledge, but then for one reason or another finds himself speechless. And sometimes his finger will curl downward as a physical response to that. I've seen it happen Ed, Edd n' Eddy, Dexter's Lab (the Computress episode especially), and Foster's Home for Imaginary Friends.