When my dd was a bit younger she'd do this to me, sometimes resulting in big black-purple-blue uuuugly hickies. Of course I'd have no clue until the morning when I'd happen to look and find a nasty suprise. She has always been a big comfort sucker, so she was likely happy to find any soft suckable part of the boob to inhale.
Me... not so happy! But I guess it beats waking up to fix it.
My dd does that, though usually she gets tummy instead of boob. Also, if she's tired or really geared up to go, she'll try latching on to my belly on the couch. That really hurts because I can't always get her to my boob before she bites (cause she's not getting any milk.)