A Few Good Men
You can't handle the truth, but the truth is, this line was an accident. The line was originally supposed to be "You already have the truth" but Jack Nicholson said this one when he forgot the line. The director was absolutely stunned and demanded the improvised line be included in the final version. It has become one of the most famous quotes in movie history.
The entire rant about being funny was made up on the day of shooting. The. Entire. Thing. From "I'm funny how? I mean, funny like I'm a clown? I amuse you? I make you laugh?" all the way down to Tommy declaring to Sonny "Yeah, alright. For $7,000, I charge you." The script just said to go on a rant, and Joe Pesci thankfully more than delivered on that front...