I have a client that sometimes forgets to breathe during exercises.  This is not helpful to her during her exercises.  We've learned that if I get her to talk during difficult exercises, she remembers to breathe and the exercise goes much better.  Her body is strong and knows how to do the exercise correctly, but her mind needs a distraction to stop her from over-thinking things!  (So, to all my Pilates teachers, I am not endorsing talking continuously during a session, just in spurts to keep the air flowing vs holding the breath.  You can't talk without breathing)