My story sounds a lot like the others. My husband and I had been trail riding for about two hours at our local park. Both my horse and I were very relaxed and I admit I wasn't paying attention. Suddenly my husband's mare shied and took off and my mare did the same. I had been riding with a loose rein and when my mare took off the reins were pulled out of my hands and I had no control. I don't remember what happened but I must have fallen off at a full gallop.
The result was four broken ribs, a concussion, and a ride in an ambulance followed by a night in the hospital. The medical people told me that I most likely would have suffered brain damage or died had I not been wearing a helmet. Now even if I am just riding in the paddock at home, I put my helmet on - it is the least expensive insurance policy you can buy. One more thing - the cell phone was on my horse. Luckily she came right back after I fell (or so I have been told since I don't remember anything). As a safety precaution carry your cell phone on you not on your horse in case you are separated.