There have been some great comments here. Someone mentioned "communication, communication, communication" and I am simply reiterating that very important comment. There is nothing wrong with treading just a bit on someone's hard limits. It is also important for a submissive to communicate to the Dominant what the hard limits are about. I don't like hearing/saying (since I am a switch) that some certain act is a hard limit without knowing WHY it is a hard limit. From a Dominant perspective, it is my responsibility to ask probing questions and fully understand exactly what the problem is, why it sets the submissive off, and what the submissive thinks will happen if I pounce on that nerve.
Along the same vein, though...some people may state a hard limit just because they are afraid of doing a certain thing. A good example is anal sex. (starts grinning already) A person may say this is a hard limit because they are afraid of it. Then they do it and it's the greatest thing ever. On the other hand, if an ex-husband forced his wife to perform oral sex and that is why it's a hard limit...that's another thing completely.
Still, allow me to pontificate just a bit further. Is it good for the submissive to forever and ever from then on refuse oral sex because one person abused him/her with that act? Remember, it is an act. It was not the oral sex that was the abuser, it was the ex-husband. It may be extremely helpful for the submissive to come to differentiate between the act and the abuser. If a Dominant can gently move the submissive through that thinking process, He/She may perform an invaluable service to the submissive.
Communicate before all else, during, and after.
"I assure you, Your Honor, I don't have to practice...I'm very good at them..."
(The Marquis de Sade at one of his trials for the sexual perversities he practiced)