I have a few "hard limits"...
Some are based upon medical issues. ie - If you touch my throat, it will almost always cause a laryngospasm and compromise my breathing. That's going to end a session anyway. So, why do it?
Some are based on the fact that they are strong flashback triggers. I generally flash at least a bit during a good tickle session. It's one of the results of tickling having been used against me in abuse situations. I pretty much accept it, brush it off and move on. But, there are some spots that, if focused on, can cause a severe flash that can actually leave me bleeding and in serious pain. Besides the fact that this is something I don't relish experiencing again, I also don't care to put a ler in the position of being scared shitless and having to tend to me after the fact if that happens.
Some are based on the fact that I only play sexually with Drew. So, breasts, butt and genitals are off limits to anyone else.
When I play with another person, I want for both of us to enjoy the experience. If the hard limits/boundaries are violated, neither of us will enjoy things. If I have a great deal of trust in a person, I may let them push certain limits a bit. But, I make damn sure that they know and are prepared to deal with the possible outcome if pushed too far. At least that way they are making an informed decision.
For me, a hard limit is not something I take lightly. I'd prefer to not need them at all. So, those I have are there for a very valid reason. If someone can't accept/respect those limits, then I simply won't play with them. Not a huge deal in my book. If someone promises to honor those limits and then deliberately violates them...? Look out! You're going to get your ass kicked!