At the end of the trial, Corinne's shocked reaction upon learning that Julian belongs to her boyfriend (Jon/Stewart/Kevin Gerrity) makes no sense. Throughout the trial, the judge makes reference to Julian as "Julian McGrath, aka Julian Gerrity" and several statements were made regarding "Sonny Koufax impersonating Kevin Gerrity" so there is no way that Corinne wouldn't have put the pieces together to realize what the situation was.
At the super market, Julian has an itch and Corinne lays a package of frozen vegetables on his arm. In the first shot, the package is right side up. But in the very next shot the package is upside down.
When Julian won't stop talking to the homeless man before going into McDonalds, the homeless man asks Sonny to get him a Sausage McMuffin. When Sonny and Julian come out from McDonalds the homeless man asks Sonny where his Egg McMuffin is.
When Sonny is walking back to couch after tuning the television and says, "Let's do it, Rangers," he places his Pepsi behind the Chunk Chip bag. In the next shot when he says, "How you doing?" to Julian, the Pepsi is to the right behind the Cheetos Bag.
When Sonny and Julian are shopping at the store, Sonny starts denting Spaghetti Os cans to save money. The second can dropped moves from the left of sonny to the right. At both times before the shot the can was stationary.
At the beginning of Lenny Koufax's examination of Sonny, he tells Sonny to cut his fingernails because they are too long. Minutes later when Sonny is hugging Julian, his hands are visible and it is obvious that Sonny's fingernails do not need trimming.
In the scene where Sonny and Julian are warning roller-bladers to "slow-down", they are standing about a few feet away from each other. When the homeless man comes by in a shopping cart, they appear to be standing further apart. When it cuts to them again (after the homeless man has passed them), they are standing close to each other again.
When the skater coming down the path in the park hits Sonny's stick, he careens down the hill into the pond between two large rocks. The smaller rock on the right lifts and floats a little to the right.
When Julian spills the milk on the floor the milk is spreading towards the back of the room. Then when the camera switches back to Sonny the milk is a circular pool before Sunny put the newspaper on it. The newspaper never gets wet.
The goof item below may give away important plot points.
At the end of the movie when they gather at Hooter's, Sonny kisses Layla and his baby. While in that shot, it is obviously a live infant, a few moments later, Layla is seen leaning over the table of food with a doll instead.