When you say mistake, do you mean a fortunate mistake like the discovery of penicillin?

Leonard

Sheldon: Hello, Amy Farrah Fowler. I'm sorry to inform you that you've been taken in by unsupportable mathematics designed to prey upon the gullible and the lonely. Additionally, I am being blackmailed with a hidden dirty sock.
Amy: If that was slang, I'm unfamiliar with it. If it was literal, I share your aversion to soiled hosiery.

Wolowitz: Okay, what if I were to tell you, tomorrow at 4:30, you could meet a woman who has been scientifically chosen to be your perfect mate?
Sheldon: I would snort with derision and throw my arms in the air, exhausted by your constant tomfoolery.

Incidentally, one can get beaten up in school simply by referring to oneself as one.

Sheldon

Yes, in 1917 when Albert Einstein established the theoretic foundation for the laser in his paper "Zur Quantentheorie der Strahlung," his fondest hope was that the resultant device be bitchin'.

Sheldon

Sheldon: As a native Texan, I must say that I've never heard the phrase "yee haw" used in quite that context.
Penny: Oh God.
Sheldon: "Oh God." That I've heard on multiple occasions.

I'm telling you, dude. The only way to feel better about Penny going out with other guys is for you to get back on the whores.

Raj

Don't worry about the moon. We set our laser to "stun."

Leonard

Sheldon: I should have brought an umbrella.
Leonard: What for? It's not gonna rain.
Sheldon: I know that, but with skin as fair as mine, moon burn is a real possibility.

Sheldon: Are you asking me or telling me?
Leonard: Telling you?

Sheldon: That's not an apology; that's simply an acknowledgment that I was right.
Leonard: Okay, I'm sorry.
Sheldon: There you go.

Leonard: Is that your dad?
Wolowitz: If she grows any more hair on her face, yes.

TBBT Quotes

Oh, Bernadette, please play my clarinet.

Raj's poem

Sheldon: I'll have a diet Coke.
Penny: Can you please order a cocktail? I need to practice mixing drinks.
Sheldon: Fine... I'll have a virgin Cuba Libre.
Penny: That's... rum and Coke without the rum.
Sheldon: Yes, and would you make it diet?