Hm let me tell you about my horse.  I had been on DIY livery for around 13 years before I moved to my own place last year.  One of the other liveries is a total nutter.  I had had Blair from 2007 and in 2009 his behaviour changed totally over a weekend, he didn't want the head collar on etc and was actually turning his bum to me when he saw it.  He had become headshy, he didn't want his face/head touched at all and didn't want us near him.  He is a very affectionate horse normally and this was out of character.  He wasn't being turned out either as it was winter and he has a rug ripping fetish (other horses', not his own) and it cost me a fortune to replace them all so I kept him in.  On the Monday I finished work in the morning and went to the farm, my husband had fed him at 0630 and he had been OK.  I turned up at 0830 for the farrier and went in with the headcollar and he tried to boot me out the stable.  He had a swelling on his face on one side which he had not had at 0630, the only person there between 0630 and 0830 was the nutter.  I got the vet out and he diagnosed a crack in the maxilla bone in the face.  The police were informed as the vet said no way had that happened just standing in a stable eating breakfast.  The nutter wasn't around the rest of the day to be challenged and as she was at work (school teacher) the police couldn't catch up with her to speak to her.  The following morning again he was sore but OK at 0630 and I got called at 1000 by another livery to say he was on the floor in agony.  I called the vet and went over, the swelling was huge and he was so poorly, the vet said it was no longer a crack but was now a full fracture, so he had been hit again.  Fortunately he made a full recovery, it turned out the nutter had been seen hitting him a day or so before by one of the liveries but no one witnessed these two incidents and in Scots law you need at least 2 witnesses or it's one word against another so she got off scot free.  Since then I've been told she was bragging about it to her relatives so she did definitely do it.  Be very vigilant as someone may be getting at your horse, not necessarily the YO but could be someone else.