Chihuahuas can have a not completely closed cranial bone (is this the right word?), which means if you pet them at the wrong place you literally can touch the brain and potentially kill them. My mood would be bad, too, if this was the case for my body.

Pugs also can have their eyes popping out if you handle them wrong (e.g. gripping them at their neck).

The head of King Charles Spaniels are to small, meaning their brain does not have enough room. This can lead to brain fluids getting stuck in the head, which increases pressure on the brain, leading to infathomable headaches, hallucinations, motor deficits, etc, pp.

Breeding some dog breeds should really be prohibited.