I've always understood it that you have 'hard limits' which can never be broken (unless there has been a discussion about it beforehand and both sub and dom agree to it) and 'soft limits' which are things the sub is a bit iffy on but which a Dom may push a little in session.

So, for me, smoking is a hard limit. I hate the concept of it, I don't want it near me during a session, I cannot see this changing and would consider it a betrayal if a Domme broke that limit. The same goes for scat and bestiality.
Electrics, on the other hand, used to be a soft limit for me and still is to a certain extent but it has got softer over time. I had this limit pushed in a session, just a mild bit of electro-play, and that has encouraged me to try it more.

So, I think an important distinction is consent and when it is given. Soft limits may be pushed a little with no prior warning - brought up during the session in a 'why don't we try this?' sort of way - so long as you are careful with it, maybe ask how they feel about it now and start the sub off slow. Hard limits should never be tested unless the sub themselves comes to you to say 'I've had a rethink about this limit, I'd like to try it' and you discuss it and renegotiate the limit.