I have never experienced it, but have heard stories from relatives who were volunteer fire fighters in small towns. How truly frightening, as the horse instinctively wants to return to the "safety" of the barn, even though you try to tell them that's where the danger lies. Trying to reason with a huge panicked equine is nearly impossible, and I know the only way is to cover their eyes and lead them strongly away as quickly but calmly as possible, and get them anchored somewhere they cannot break free from. The more horses involved, the more impossible the task gets, because they scream in fear and continually panick each other. It is a terrifying phenomenom, and my heart wrenches for anyone who has been through it.