Continuity mistake: During the wedding reception, George (Steve Martin) is called outside to meet with the police because there are too many cars parked in the street. Notice that the police car is curbside completely surrounded by cars. In fact, the whole street is packed with cars, making one wonder how the police car got up to the house.
Other mistake: When Franc (Martin Short) is looking at the house to see the changes for the wedding and they are talking about the menu, Kimberly Williams starts with her hair in a ponytail, then for a split second her hair is half up in a barrette and then it's back to a pony tail (or vice versa).
Continuity mistake: During their first visit to meet Franc George, Nina and Annie are sitting on the couch. When they are first discussing the date of the wedding and the camera is focused on them you hear Franc close his appt. book, but when the camera goes back to Franc you hear the same sound as he closes the book again.
Continuity mistake: When the bride-to-be, Annie, runs in the front door upset, George and Nina, her parents, are standing in the dining room looking through all of the tacky presents Annie and Brian have received. George is holding a silver tea set on a platter. In every shot that flashes back and forth between he and Annie, the teapot seems to turn itself in a different direction on the platter.
Continuity mistake: When Nina is sitting at the table during brunch with the in-laws (and George is running around the pool outside) in the wide shots her hands are up under her chin, and in the closeups they're flat on the table.
Continuity mistake: During the scene where Franc and the other consultants are planning the wedding in the Banks' living room, Steve Martin tries to show the construction worker how to open the French doors. The jacket of his (old) tux splits as he is struggling with the door. If you watch the scene is slo-mo, the back seam of the jacket splits as Steve Martin is bending down to reach the door knob, the seam "reconnects", and then splits again, the second time with the ripping fabric sound effect.
Continuity mistake: When the Bank's first arrive at the MacKenzie's to meet them, they have 3 dogs. For the rest of the movie, and the sequel, they only have 2. (00:28:40 - 00:29:10)