I can't believe no one has brought up David Carradine as an example of going too far.
If I bolt for the door, that's a good indicator
I've always thought going a little too far is an important part of any play.
Expanding your boundaries.
Going too far makes the cuddling after play just vital. That's the time when you're going to get the best feedback so you can make the next time better
As far as humiliation goes, it's a lot more than just calling someone names. Just being naked with a fully clothed person is humbling. You can expand on that in all kinds of ways without ever a discouraging word. (like wearing a nun outfit

)
I bet EW looks hot in her habit. (sorry about the pun)