My sister had a dog named bandit that they couldn't do a thing with. I visited once a month, since I was only 2.5 hours away, and never had any problems with her. She would walk to heel without a leash, fetch, etc... Even for them when I was there.
One time I left one of my shirts behind, and she appropriated it for her bed. They didn't have any issues until 6 months later when Bandit disappeared for 2 weeks, and came back home having eaten a plastic bag that tied up her guts. No operation possible, they had to put her to sleep. When they buried her in the back yard, I insisted that they nest her in my shirt. I still get sad thinking about her.
I had a dog from 12-18 that I could still cry about if I dwelt on it for long. I loved that dog so much.
We were like best friends.