It probably sounds silly but this happens more often than you think, especially a bad experience can have some serious consequences. Which is why you don´t want to put too much haste into your first session, it´s important that you know what you get yourself into and that you at least have spoken with your Dom/Domme before starting a session. Once again this seems silly but I´ve heard some strange stories, people that get themselves into a mess and regret ever putting themselves in that position.
Also not knowing what to expect might seem hot but it´s just stupid, it can lead to confusion or unpleasant situations. For example someone that expects sex might be in for disappointment and those that thought they would get a slap on the wrists might be in for more then they bargained for. Once again it´s important to communicate, who is your Dom/Domme and what does this person want? What do you want? What interests you? But also what do you expect to happen in a session? Questions that can help you more than you know, especially if you´re new to BDSM!
But what if you´re a Dom/Domme and your first session is a disaster, does this make you a bad Dom/Domme? It doesn´t have to mean anything, it really depends on you. If you´re nervous then that´s not a bad thing, I think most Doms/Dommes were nervous in their first session and some might still be nervous just before a session (especially if it´s the first time with a new sub). There is nothing wrong with that, but don´t let that control you and just stick to what you know. Don´t let yourself be tricked into doing something you don´t want to do or something that you´re not familiar with. It´s nothing to be ashamed of to say no, no matter what the other might say.
If you feel uncomfortable in the role of being a Dom/Domme then it might be a good idea to ask yourself if this is the right role for you. Because not everyone is cut out for that, which doesn´t mean that you can´t be a Dom/Domme but perhaps it might be smarter to put yourself in a different role. Maybe you´re better as a switch? Which doesn´t make you anything less! Once again rushing yourself into a first session usually doesn´t work well for you, find out what you want and what you like. Then put that next to what you know and what you´re capable of, that should lead to an interesting outcome!