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: "How do you like those apples" is an expression used to denote triumph, like "told you so" or "put that in your pipe and smoke it". Will just adds his own little humorous twist to it.
Grumpy Scot