In the beginning when Julia is dancing over the vent, you can clearly see she is wearing black underwear. Later when she is sprayed with the hose and knocked over, she is wearing flowered underwear.
At the end when the carrige is going away. You see marks from the other takes before that.
When Julia and Grant are talking to Jello about planning their wedding, the position of the door behind Julia keeps changing from open to closed.
When Grant, Julia, and her parents are getting out of the van, Grant is in the background, throwing the luggage on the street. When Grant's Dad knocks out Julia and her parents, only one piece of luggage is on the ground.
When Bernie toasts Julia and Grant at their wedding rehearsal, the level of water in Roz's glass changes without her touching it.
Many small errors are probably left in deliberately to fit the absurd nature of the movie.
When Julia dances for the firemen, they hit her with the fire hose, knocking her back. In a few shots afterward, especially in the close-up of her swishing her hair back, her neck-fat prosthesis has come loose.
Right before the rehearsal dinner, when Andy does her cherry stem thing, the carousel slows down. A split second later, in the next shot, it revolves quickly again.
When the camera man falls on the "Wedding Crasher," he clearly falls on a dummy.
In the motorbike scene, it can be clearly seen that the stunt woman's skin is much darker than Julia's.
When Andy is talking to Grant, just after he meets Julia, they go to sit down and she takes her whole bikini off. She is supposed to be fully naked, but the strap of a flesh-colored thong is visible.
When Julia is doing her "Milkshake" dance, she starts dancing to the taxi driver. In a side shot, the camera is reflected in the side of the taxi.
Julia's heritage is Japanese, Indian, Black, and Jewish. When Roz meets Julia, she says that she has been waiting to meet the "shiksa," which is Yiddish for a non-Jewish girl.