I found one in the park a few years ago, walking my (female) Border Collie. There was a man walking towards me with his dog on a lead being followed by this big white with brown spots male American Bulldog. I gave him the usual "hi" greeting - he said "this dog keeps following me and I don't know what to do". Yikes! I always keep a spare lead on me (how many times have I gone out with my dogs and come home with additional lost dogs, don't ask)! So I walked my dog and this American Bulldog around the park for about an hour, hoping to come across the owner. The looks on peoples faces I remember very well - they'd see the dog and almost recoil in horror. But he was very well behaved and enjoyed the walk. No owner, so took him home where he tried to eat my sofa (don't think he'd been allowed inside a house). Then climbed on my (startled) 20+ aged niece who was living with me and snoozed. I posted on Facebook local neighbourhood - my dogwalker responded saying the dog probably been dumped (said he'd found some himself in the past). I put the big lad in the car around 9pm and drove him across London to Battersea Dogs Home. I didn't want to take him to a police station (anyone remember the story of a lost dog being taken to a police station, put in kennels and forgotten - found days later dead, horrible, really horrible). Had a reluctant night staffer take him in - I said I'd walk him to a kennel if it helped, which the staff was relieved to hear! The big lad was reluctant to go in but didn't give me any trouble, looked at me with his big sad brown eyes. I kept in touch, Battersea said the dog wasn't chipped, had bad urine burns on his feet from probably being kept in an enclosure with no access to outdoors, and 10 days later he was destroyed, Battersea saying he showed guarding tendencies around food (he'd been fine with my female dog at my home). Have to say, it pretty much destroyed me - I was dreadfully upset, I'd taken him there and felt responsible.
My MIL's minature dachshund bit the postman. But a dachshund jaws aren't the same as an American Bulldog jaws!
I'm firmly of the opinion if there's a problem it's the owner more than the dog, but would agree some breeds really are not suitable for domestic situations.