In middle school I used to kick the soccer ball around a lot in the yard after school. Sometimes I'd kick it as high into the air as I could and then head the ball (with incorrect technique) on the very top of my head.

After I headed the ball I would see what looked like a cloud of black dust in my division for a few seconds and I remember at the time thinking it was kind of interesting. Is that "seeing stars" or something else?

Probably did that at least 50 times or more in total and some days I remember doing it a few times in a row. Don't know if they were concussions, but it wasn't a good move. And the soccer balls I used were usually either underinflated or overinflated; they were also often covered in water. Never did use proper technique with the forehead since our soccer coaches tended to be parents who had never played soccer before. And so even in practice and games, I always hit the ball with the top of my head and never knew then of potential dangers.

Definitelty don't hit soccer headers, even with perfect technique.