It's very rare but there's still tons of documented cases.
Here's a recent one.
It's funny people who say it's a hoax. They'll say there is no such thing, then faced with proof that a body burned on its own without burning much of anything around it, they'll say "Oh well, they just fell asleep smoking." or "they were an alcoholic and had so much alcohol in their flesh they were combustible". Neither of those are possible explanations though.