When Jack begins climbing the beanstalk, the words "Jack begins climbing the magical beanstalk" appear in the book of Jack's story. When Jack climbs down to see for himself, the page goes blank again (at around 53 mins), but as Jack turns the page to see the next page, we see that the original page was not blank, but was the "Once Upon a Time ..." page.
When Jack begins to talk alone with the headmaster, the C.O.W. (computer of wonder) lies on the headmaster's podium (at around 11 mins). When Jack leaves the classroom he takes it from his own desk.
As students exit the classroom, Sleeping Beauty awakes then stands up, while Rapunzel fetches a trolley carrying the rest of her long hair. However, in the next shot Beauty is still asleep at her desk, while the trolley & its hair have been left behind; then suddenly both girls & the trolley of hair are leaving through the door.
Gilbert Gottfried's first appearance occurs as he passes a window and sees his reflection. The candle on the window sill is on the same side of the window...outside.
In the pillow-fight scene, the stunt double for Jillian (Chloe Moretz) receives several close-ups where the face is clearly seen to be Asian.