The thing I'm intending to submit to the Angry Robot open door started as an opening - I had no idea where it went next and certainly no vision of an ending. I didn't get that until I was about 60-70k in.Don't most pantsers, like Steve said, have a rough outline in their head, even if it's not all meticulously laid out on paper?
The one I finished writing a few weeks back had an opening and an idea of an ending, but it all died in the middle for a while - then I did what anyone would do, and started a thread here on pantsing until I could get that discovery mechanism to do something useful and deliver an answer.
Having said that, surely the point is that if all you have is a vague couple of bullet-points in your head, you're a pantser. If you have the whole thing mapped out in fine detail, you're a plotter. Anything in between makes you a strange hybrid who gets to pick their own label.