Yes, it happens, however there are a PLETHORA of other dog breeds that can and do the same, you just don't hear about them because the media loves stories on Pitbulls. When I was a kid I grew up next to a 190lb German Rottweiler that attacked HIS owner, and I was afraid of Rottweilers for years.....Hello; Well, yes they are. The numbers are that something around 60+ percent of fatal maulings are done by pitt bull types. I have some of the links saved if you wish to see for yourself. It some of the maulings the dogs were raised from pups and killed their owner (the person who raised them.) Fatal maulings of children are a considerabl portion.
......only to grow older and realize that I was being silly for hating any breed/race/species just because of one instance. Food for thought, the Cocker Spaniel is responsible for more dog bites each year than any other dog....my Vet puts a muzzle on them IMMEDIATELY regardless of how "nice" they are.
You never hear about them though, because they don't have 2,000lb. PSI jaw strength and don't cause that kind of damage. Bad owners or bad breed?