I learned this lesson the hard way today. My wife & I grew up with horses but got out the habit. We bought our horses in the fall and have had a great time. My horse is a 'husband horse' - a 17 year old quarter horse gelding. He knows his stuff, is close to bomb proof and knows his way on a trail, sure footed and precise.
Today it was fantastic weather and my wife and I went on a long ride. We ended up about 5 miles from home riding through a meadow of grass with scatted clumps of scrub oak. As we rode past some oak it happened. My horse suddenly did a 90 degree bolt to the left at top speed kicking his heels. If I had remembered my lessons of 'keep a good seat' and paid attention I probably would have made it. I was leaning back in saddle and holding the reins with one hand. I didn't even have the stirrups adjusted long enough since somebody else had ridden it. Bad. Bad. Bad. I'll never know what was there but there are mountain lion around.
I had no chance to lean in and just tumbled back. It's amazing how things go in slow motion at a time like this. As I went over the back I had a thought that I wish I'd installed the safety stirrups I got for Christmas. Then I saw my legs pop loose from the stirrups and then a thought, 'This is going to hurt!'. I landed hard but the ground was pretty soft. I wiggled and everything moved so I got up. My wife's horse had bolted too and she'd gone down. We both have a good set of bruises and strains but it could have been much worse.
The horses ran away and we found them grazing in a nice spot a mile away. They didn't have a single scratch or cut. One rein broke but the saddles weren't even scratched. My horse, being older, is rather sore now but his gait shows no strains, just a tired horse.
We were amazingly lucky and learned a good lesson. Horse back riding is relaxing but don't relax too much. Make sure the tack is adjusted right and be ready for the unexpected.