Deliberate mistake: In the first scene Velma gets sprayed by the hose and slips but gets her foot caught on the chain; hanging upside down, but her skirt doesn't go "up". Deliberate to maintain the "family movie" tone. (00:02:40)
Deliberate mistake: When bars are placed on Harry's windows, so he does not escape, it is apparent that his windows have complete leaded panes of glass. Yet when Harry is being rescued, Ron and Harry speak with each other, even in low voices, through his visibly closed windows. (00:08:00 - 00:09:00)
Deliberate mistake: When Scrat's coconut falls from the tree, it has already had its husk removed. The directors confirm this on the DVD commentary. They may have been worried that a lot of their audience would not recognise a coconut with its husk on. (01:11:45)
Deliberate mistake: In multiple shots, Stitch's ear notches move higher in places or vanish completely.