Continuity mistake: When Gunner is running under the table towards Hank, on the left we can see Henry's and Jake's legs as if they are sitting at the table. But neither Henry or Jake are sitting at the table at this moment, we see them come into the room a few seconds later. (00:38:05)
Continuity mistake: When Hank is being attacked by the dog and the family is looking on and laughing, it cuts to Charlie laughing and he is holding a glass to take a drink, then it cuts back to Hank, and just a second later it cuts back to the family at the table and Charlie is sitting with his arm across his chair facing a completely different way, with nothing in his hands. There is no way he could have put down his glass and turned to lean back with his arm across the chair in that short of time. (00:40:20)
Continuity mistake: When Steve Martin is talking to Bonnie Hunt about being gone for three days, watch the picture on the table. It moves closer to the edge of the table in the subsequent shot. In the first shot it is not covering the ring on the edge of the table, in the next shot it is. (00:43:10)
Continuity mistake: After Mark has been bullied, we see the bullies walk away and do their handshake, but in the very next shot (from a different angle) they do their handshake again. (00:43:35)
Continuity mistake: When Bonnie pulls away from her house in the taxi to go to New York, there are no pedestrians on the other side of the street. In the next shot, just a moment later, two people are standing on the previously empty sidewalk facing the departing taxi. (00:43:50)
Continuity mistake: When Mark is walking down the hallway on his first day of school, he runs into two bullies. After the bully grabs Mark's glasses he then pulls Mark's hat down. Then, when the other bully is about to pull Mark's hat down the hat is up and perfectly fine. (00:45:25)
Audio problem: When Steve Martin is talking to Bonnie Hunt on the phone and the kids are playing, a small dart lands between his feet. The sound the dart makes when it sticks to the floor is not accurate at all and deliberately sounds as if it comes from a object much heavier. (00:50:25)
Character mistake: In the sequence when Tom is on the phone trying to find a babysitter, he repeatedly says he needs a babysitter for 12 children. However, Charlie, Lorraine and Nora, who does not even live at home, are much too old to need a babysitter. (00:50:45 - 00:51:25)
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)
Continuity mistake: When Charlie is doing his homework, he throws his pen on the book on his bed and looks at the picture of his girlfriend. He then picks his pen up from the book in his lap. (00:53:35)
Continuity mistake: When Tom is in an office at the Stallions, Charlie comes in and he says "Dad we need to talk" and then he picks up a rugby ball with one hand. In the following closeup though he suddenly is holding it with both hands. (00:53:35)
Continuity mistake: When Tom picks the kids up from school, Mark hands his dad a picture. In a closeup Mark is holding the piece of paper at the top, yet in the wideshot he is holding it at the side. (00:54:30)
Continuity mistake: At school Mike Baker knocks the latte out of the bullys hand, and the latte falls to the floor spilling a bit of drink on the bullys shoe. Yet in the next wideshot of Mike lunging at the bullys legs, the drink cup has vanished and both of the bullys shoes are clean. (00:58:55)
Continuity mistake: After the scene where the kids get grounded, there is a scene of Mark laying in bed. He then rolls over in bed and stares at a picture of him and his mum. In front of the picture are his glasses, but in the very first overhead shot of Mark laying in bed, the glasses weren't in front of the picture. (01:01:00)
Continuity mistake: At the hospital, Dylan has a black eye and broken arm after Tom fell on him at his birthday party. A little later when Kate goes to the Shenk's house to tell them Mark has run away, Dylan shows no signs of the injuries he suffered. (01:04:45 - 01:18:25)
Continuity mistake: When Tom and the family are getting ready for Oprah to come round, Tom is on the phone to Kate and he runs over to a vase of yellow flowers and nearly knocks them to the ground. Between shots the way he touchs the flowers changes. (01:08:50)
Continuity mistake: When Hank and Nora come to visit, Hank is walking on the sidewalk leading to the house, and we see him take off one of his gloves. In the next shot, he takes off the same glove. (01:10:00)
Continuity mistake: When Mark spills water on Tom, Tom has a big wet patch on his trouser leg. This disappears when he goes upstairs. (01:13:20)
Continuity mistake: When Nora and Hank are standing by the bainster, when the whole family is fighting, in one shot Nora's arms are down by her side, but in the next shot, her hand is up by her chest. (01:15:35)
Continuity mistake: When Bonnie Hunt announces that Mark has run away, she has no paper in her hand. It cuts to Steve Martin, then back to Bonnie who now has Mark's note against her chest. (01:17:05)
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).