My dog never actually shit on me. But once my families dog had its period and we made it wear period pants, it was clearly not happy about it and constantly tried to get them off, but no success was had even though it rubbed itself against anything it could in the living room. But then while it was standing there in the living room it looked, with its back to us who were all kind of laughing at its poor attempts at getting off the pants, its legs started vibrating. You could see all the muscles tightening in its body and then it happened, it was shitting with its pants on. To demonstrate :
It was possible the nastiest thing my dog has ever done and it has done some nasty things, but the dogs plan worked and we had to take the pants of it which at this point was like a full diaper, it was already starting to come off the dog cause the pants were that full of shit, even though a good portion had already gone out of the sides and onto the ground.