:o Could be you're right!
I've clearly missed out... I've found mutts, Labradors, Collies, a couple of Rotts but never a Jackie.
One of the Rotts was good fun. I called the Dog Warden out to scan her for a chip. He duly arrived and as I led him to the kitchen where she'd been eating biscuits from my hand he asked me "Is she friendly?"
"Yep... well, she is to me at least. I hope she likes men!"
Whoops. At that he opened the door and she flew at him, hackles raised, teeth bared! How was I to know my wisecrack might have more than a ring of truth to it? 
The Dog Warden, a big, burly chap, slammed the door shut PDQ, handed me a slip lead and said, "Can you get her into the van then?".
She walked out by my side and hopped into the van with all the gentleness and manners of a PAT dog. The DW sat in the drivers seat, doors and windows firmly closed.
A few days later, chatting to my local greengrocer, I mentioned finding this Rott. "Oh, so it was YOU! She's my dog. Thank you!" she replied. A visitor had left the door open and Rott-Girl had escaped, leaving the owner frantic. Luckily the microchip had reunited them.
I smiled and told her owner of the difference between Rott-Girl's reaction to me and the dog warden. She snorted. "My dog hates men. I trained her to be like that in case my bastard ex husband comes back!".