Deliberate mistake: In the scene after the Helm's Deep battle, when Aragorn meets Eowyn, the earlier footage of Aragorn's return to Helm's Deep is used. He has the wound on his shoulder and is dressed like he was during the 'falling' episode. The background (people in the fortress walking calmly) also indicates that this takes place before the battle. This scene should immediately follow the one where Aragorn is greeted by Gimli ("you are the most reckless man I ever knew"). Confirmed on cast commentary, extended DVD. (02:45:20)
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)