The stunt where Jim Carrey (Charlie) throws himself out of the car was done in only one take.
The glasses that Michael Bowman (Casper AKA "Whitey") wears are his real-life glasses, including the little microscope on the end.
During the "Hank Breastfeeding on the MILF" scene, Jim Carrey unknowingly latched onto Shannon Whirry's right-nipple; he thought she was wearing a prop at first, until he tasted it and quickly found out that they are genuinely her lactating ample breasts. Carrey explains: "It's very rare that I get to a place where I am actually kind of humiliated, but that was one of them. It was so embarrassing to me, I was like embarrassed for the girl, but she was fine with it. She was like 'Yeah, whatever.' But I was literally apologizing to her between every take. When it was time to film it, I had to empty the studio. I couldn't do it in front of anybody." This is surprisingly ironic, because Carrey at other times would openly admit his love for big-breasted women; while Whirry has no qualms about doing topless scenes and "wet-nursing stunts", because ever since one year after the film's script was completed, she had been playing in over a dozen erotic films in her acting career.
Jim Carrey ad-libbed the moment when he asks his sons to kiss him before he leaves. One of them looks obviously surprised.
In the scene where Charlie and Irene are pushing the car off the cliff, Jim Carrey says "On nine" and counts to nine very fast. This was ad-libbed, and for a moment, Renée Zellweger looks obviously surprised.