Firstly, this is my round pen:
As you can see, it's made of very thick posts (these are usually used to hang gates and to set the tension on high-tensile wire fences) and rails. It's a very good round pen and I used it for over a year, with horses from 12hh to 16.2hh and temperaments from quiet to troubled, with no issues.
However, a couple of days ago, I was round penning my filly, and she decided she wasn't going to go in the direction I was asking. I blocked off her option to go forward, allowing her the option to turn around or cut through the middle of the round pen. She took neither, instead trying to jump out, but took off far too close and couldn't get the height in time. She caught her front legs and went over the top, breaking one of those huge posts. And, like I said, I suspect she may have a hairline fracture in one of her neck vertebrae - either C1 or C2, the swelling and heat is very close to her head.
What is wrong with this round pen is that the fence is not tall enough, so she thought she physically COULD jump out, and the round pen itself is not large enough at only around 22ft diameter. It's the perfect size for ponies, and a large horse can w/t/c in it in a balanced manner, but it's too small for a large horse that's in panic mode.
So please PLEASE ensure your round pen is fenced with strong fencing at least 7 foot high, and is at least 50 feet in diameter. My girl could very easily have died the other day... I would hate for this to happen to someone else. Magic and I were incredibly lucky that it wasn't worse, and every time there is an accident similar to this there is a very high chance of serious injury or even death for the poor horse involved.