Do you ever wonder why horses continue to spook at things they have seen a hundred times before? It’s because they have a hard time generalizing objects. Just because a large, pink floral arrangement by the dressage arena didn’t eat your horse last week, it doesn’t necessarily mean that a small, yellow one might not do it on Saturday.
Horses are inherently skeptical of new things – we all know that. But a rider could be forgiven for thinking the horse might be "doing it on purpose" when for the umpteenth time, a dressage test or showjumping round is ruined by inappropriate displays of avoidance behaviour.