It doesn't matter as to how safe you think your horses are they will always find something to injure themselves on!
I helped to bring a two year old colt in from a field after he had galloped into a post and rail fence and impaled himself through the chest so the rail came out through his ribs by his elbow.
My yearling's grand dam impaled herself so that her heart was actually visible looking through her chest. She lived and foaled later that year.
This was also done on a gate latch which was on the outside of the gate.
I am very safety conscious and this was a freak accident. Around and about I see horses kept in poorly fenced fields with loose barb wire, chain harrows and other rubbish left lying around but they never seem to injure themselves.