I'm having a bit of trouble playing this out in my mind. The tin was behind the trough, but suddenly the tin is outside the paddock. Does the trough border the fence and in saying the tin was behind the trough you mean it was on the other side of the fence?
IMO, if there is something hazardous to a horse within the boundaries of the pasture, and the BO has been alerted to it but chooses to leave it there and a boarder's horse is injured, then the BO must accept full responsibility.
If the hazardous thing is outside the pasture and a horse somehow gets into it, then I wouldn't count on being able to hold the BO legally liable. Frankly, I wouldn't board at a place that left junk lying around.
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