I would personally find it very difficult to inflict serious damage on a sub.
She's given herself to you, she is "Your" responsibilty. Any Dom worth his salt will surely take that responsibilty seriously.
It doesn't take much common sense to realise when things have gone too far or are approaching the limits.
Some people prefer not to use safe words anyway - where does that leave them? It's trust and care we're talking about here.
Inflicting serious damage is the sign of either gross stupidity or having a "misunderstanding" of the person you may be with. They have given themselves up to your "care" and you should "care" for them.
Fantasy obviously fills the "extreme" void for most of us.
Those individuals who cause damage in real life, because they desire to, have a problem.