Almost 4 years ago, I was riding a pretty quiet horse named Doc. He was being great for me. Then at the end of the lesson, while I was dismounting he started to buck and he probably would've bolted if someone wasn't holding him as I dismounted. My helmet flew right off of my head, so it obviously did not fit correctly and I knew that before I even mounted. I just thought, ehh it's alright. It happened pretty fast, all I remember is hanging on, being bounced around and watching my helmet fly over to the other side of my horse. I had some saddle burn on my elbow as well, and the saddle horn jabbed me.
What I'm saying is make sure that your helmet fits properly and don't just think that if it's loose that it will still be functional. It was one of the many helmets used for lessons at that barn (which I no longer ride at). In fact, all of their helmets are in poor condition with cracks in them and such, and they don't clean them... yuck. They should definetly invest in some new Troxel helmets. Also, you shouldn't use your barn's helmets, you should really get your own.