Sure they happen and, sooner or later, loose horse accidents generate considerable and sometimes multiple bills somebody has to pay. If it happens at a show, potential for a loose horse is greater and somewhat more understandable but when it happens at the home barn, its inexcusable, especially repeatedly and elite level competition horses or breeding stock can do just as much damage as that $300 auction find or a retiree.
Spoken from personal experience. Decades ago paid for a neighbors decorative picket fence, sod restoration and a few rosebushes when boarding privately, chief suspect homeowners 12 year old who was sent out to simply push a flake of alfalfa through the bars into the pipe corral. I moved the horse.
More recently, fellow boarder at a commercial, full service boarding barn had to replace a headlight, grill and radiator on top of a rather large vet bill and 6 months lay up. She was not at the barn that day, latch was secure and not accessible to horse in stall so whoever entered that stall to muck, water or feed or fool with the horse did not close it properly and night check missed it or failed to do night check. No litigation resulted and believe two barn staffers were fired.
Worst case was sometime farrier’s two horses got out of his big, very rural pasture not visible from the old farmhouse via a section of fence collapsing (or getting pushed over) in a bad, midnight T-storm. Luxury car was totaled, one horse killed, the other banged up pretty bad, later put down. Driver ( IIRC an MD specialist) very seriously injured, hospitalized for almost a month, unable to work for some time.
Not only did they go after the sometime farrier but investigators determined the fence was rotten and farrier was going back and forth with landlord for months about who would fix the fence. Both parties had knowledge of its condition so both were held at fault. Farrier split shortly after the wreck, stiffed the vet and renderer as well as landlord for several months back rent. That one was in court for some time, no idea what the final settlement was.
Anyway, I don’t make this stuff up. Loose horses should never be shrugged off as “ just” an accident. You have to assume they will get loose and take steps to contain any escapees, like perimeter fencing and gates in good repair. OPs BO is aware horses are getting loose, easily be found negligent. Because they are.