Well, again. Neural networks are very dumb.

In this case the selection bias was, that most of the dog photos were dogs playing outside in parks and other outdoor locations, while the cats were mostly photographed inside. So, as they tried to show of the model, they learned that what the model had now internalized was: "Animal outside is a dog, animal inside is a cat." And it would very confidently say this about any animal. Rat? Outside its a dog. Inside a cat. Horse? Yeah, I see plants in the background. Pretty sure that is a dog!