In the episode 1x07 titled "The Dumpling Paradox", about seven minutes in, Sheldon says, referring to Leonard:

"He's engaging in reductio ad absurdum. It's the fallacy of extending someone's

argument to ridiculous proportions and then criticizing the result."

Actually, what Sheldon was trying to say was argumentum ad absurdum. Reductio ad absurdum is no fallacy, but instead a very much valid method of disproving propositions.

Sheldon certainly should have known better.