Continuity mistake: After Mark is found, he runs into Kate's arms and hugs her and her sunglasses fall off her head, but then in the very next shot they're back on her head perfectly. (01:24:50)
Visible crew/equipment: When Tom and Dylan are hanging from the chandelier, there is a shot of the ceiling showing the chandelier coming loose. Then when it cuts back to Tom hanging from the chandelier, in the background behind him is a crewmember holding a boom mic. (00:24:15)
Continuity mistake: At the beginning during the breakfast scene, the eggs splatter all over everyone and some gets on Steve Martin's chin and you can see it fall off, they then show a shot of FedEx and then show Steve Martin again and the egg is back on his chin. (00:10:15)
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)
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).