Revealing mistake: When Matt Damon and Minnie Driver are sitting outdoors at Au Bon Pain in Harvard Square, Matt is telling her how he is able to "see" mathematical equations, just like Beethoven was able to "see" the notes on a piano. At one point during the conversation a girl wearing a red backpack (possibly a Harvard student) walks by in the background, stops and looks at the camera, smiles, and then continues on her way. (01:21:10)
Revealing mistake: When Will and Skylar are on their date and eating hamburgers, an extra in a blue jacket is seen walking by in the background multiple times. (00:45:20 - 00:45:50)
Answer: There are two versions of the joke actually. The original which is found on the DVD and then the made for TV joke. I don't remember how that one goes and it doesn't seem to appear on the DVD I've got, but the original joke is: All right, there's an old couple in bed, Mary and Paddie. They wake up on the morning of their 50th anniversary. Mary looks over and gazes adoringly at Paddie. She's like, "Oh, Jesus, Paddie. You're such a good-looking feller. I love ya. I want to give ya a little present. Anything your little heart desires, I'm goin' to give it to ya. What would you like?" Paddie's like, "Oh, gee, Mary. That's a very sweet offer. Now, in 50 years, there's one thing that's been missing, and, uh, I would like you to give me a blow job. I would like for it." Mary's like, "All right." She takes her teeth out, puts 'em in the glass. She gives him a blow job. Afterwards, Paddie's like, "yeah, geez, now that's what I've been missin'. That was the most beautiful, earth-shattering thing ever! Beautiful, Mary! I love ya! Is there anything that I can do for you?" Mary looks up to him and she goes, [Skylar takes a swig of her drink] "Give us a kiss." [And her drink comes out of her mouth, indicating what would be coming out of Mary's mouth in the joke].
Nikki
Even funnier is she has to have a Guinness or a stout so what comes out of her mouth is really dark.