When Does A Dom Cross The Line To Abuse?

[Alex Bragi]

...
If you get your ass spanked more than you thought, or liked, sometimes you have to accept that maybe on some level, you deserved it for being a bit extreme in your own behaviour. (from a sub's point of view here) You get up, with some tears and are held by your Dom, but your defiant streak/ ego is learning where to/not to go. That's teaching, but to outsiders, it could be termed abuse right?

I think, perhaps, you've unintentionally described exactly when a spanking, or any other kind of bdsm play, does becomes abusive (or an abuse of power). Of course, it's just my opinion, but does anyone, male or female, really 'deserve' to be pushed beyond what they're prepared to give freely? When the scales of play are tipped too far, isn't the balance of power exchange lost?