There will always be animation errors.
Animators don't care about some minor errors with the background ponies. Lyra could suddenly be a pegasus or Bon Bon a pegasus.
Unless it's a recurrent character, then I'd say it's a major animation mistake.
Now, if you're talking about the show's canon. Why not?
Twilight grew wings, but only because of all that princess and destiny crap. I'd say it's possible, but it would very extremely special cases.