We have a collie that we took from a rescue centre. He is affectionate, loving and gentle in the family environment and absolutely bomb proof with our children. He is very well trained, walking on heel, listening to every command. He came to us drastically underweight and we have got him looking great. But, the homing centre did not pick up that certain things trigger attacks from behind. He flinches when feet are close by, like he's been kicked. In two months he has gone for two men, a relative when we thought he was just settling in, and the latest a workman on saturday. We understand the traits of collies, we know a collie nip when we see it, but this is something else, it seems to be motivated by fright but its a full on bite. On saturday the man was bitten three times in two seconds. On each attack the alpha male (me)has not been present and he's attacked the hip, or back of the leg of the two men concerned. He can be really inconsistent with men, sometimes he can be fine and other times he's not- some men he likes- others he growls at. On the two occasions he has attacked, he was played with gently by the men concerned and the attack came by way of ambush when they turned their back. It seems tragic to us that such a fine dog has this unidentifiable trigger and we are hoping that somebody can give us advice as to how to proceed or we will not be able to keep him. Its worth saying that on most visitors coming it appears to trigger excitement and leads him to trigger play with our other collie.