I did hear about a vet who, some years back, had a slight mishap with one of these humane killers at a horse event which was open to the general public. I forget all the exact details but basically a horse got injured and it was decided that it needed to be put down. Embarrassingly for the vet, it was said that he managed to miss the horse but did manage to either shoot himself or a member of the general public; I forget which.