Other mistake: When Steve Martin is seated at the table trying to find a babysitter, there are several quick shots of the phone book as he crosses out numbers, turns the page, etc. The very last shot of the phone book is backwards! While its easy to see the phone book, if you zoom in on the shot, you can clearly see that the tiles, words and letters on the board partially obscured by the phone book are backwards too. (00:53:20)
Other mistake: At the beginning of the film, Lorraine is in the bathroom while her sister is banging on the door outside. Notice how blonde her hair is as she is applying her lip gloss. At the breakfast table a few minutes later her hair is a different type of blonde. This is because Hilary Duff wasn't available at the time of filming the beginning of the movie and her bathroom sequence had to be filmed many months after the filming for the rest of the film had been completed.
Other mistake: There are four days when Jake, Mark, Kim, Jessica and Mike are shown going to school in Chicago. All five wear the exact same clothes on the first three days, only on the fourth day are they dressed differently.
Other mistake: When Kate is telling the kids that their father is losing his job, Jake Baker is seen in the foreground, but in the next shot Sarah Baker is forward and Jake is behind.
Other mistake: When Jessica and Kim come down the stairs and Jessica is explaining what happened to Kim with Nigel and the dart, Kim looks at the camera.
Other mistake: At the end of the movie, Steve Martin is walking through the field where, just minutes before, he coached his last game (he looks at the scoreboard and his wife tells their children that he went alone to the game). Well, that seems a very immaculate football field for one that just had a college game played on it. (01:28:55)
Other mistake: When Kim and Jessica run into the kitchen at the start of the breakfast scene, Jessica looks up at the camera. (00:04:45)
Answer: It seems that the fact the family can't take care of themselves is added to part of the humour of the film. 12 children is a lot, and think about all those rows and all that mess.
Hamster ★
It's not like they couldn't take care of themselves. As shown with the breakfast scene at the beginning, they seem to function pretty well when they all get along. Tom and Kate were just usually home more frequently than some other parents. It also didn't help that it was the first time that one of the parents was gone for an extended period and the parent that was left home (Tom) had never had to deal with all the kids on his own. Also, the only time we see complete chaos in the house is the first day that Kate has been gone. After that the kids are more behaved (at home).