Did the horse spook, or were you already making the horse nervous placing it on edge and making it susceptible to surprises?
Something important to think about is the cause and root of a horse's behavior. Often it seems that people put the blame on the horse by defining it as the horse's weaknesses instead of realizing that it is in fact their own weaknesses causing the problem.
A team is only as good as its weakest link. Be careful how you think of horse's behavior.
I've even seen the most beginner-level and consistent horses spook when someone is overly neurotic, passive, continually wavering, indecisive, meek or otherwise not very good at human-ing.