Thought a minor thing, there is 1 shot I just saw that shows, in the background between Applebloom and whatever the cutie mark crusader unicorn's name is, shows applebloom's friend (wears glasses) from the first episode she and applebloom appeared in. It shows, whatever her name is's side with no cutie mark. She's standing sideways. I guess stuff like that happens.
I know on some animated shows they do happen, like on superfriends, with gloves vanishing and other stuff, On thundar he's not suposed to have his sun sword but it's clearly visible on his bracelet like normal, and other stuff on other shows.