Sorry, My Mistake!

I preach that it is simply too easy to cheat these days with the invention of the SMS etc etc.... But when these mistakes are made, even if its not cheating, are there circumstances to which forgiveness is allowed?
Are mistakes simply a point of reference to where our boundaries lye? Simply its when we have taken it too far and offended another person, so in a way this statement is true. Some boundaries are just known as a general rule so over stepping them is even riskier, but some are up to the individual so therefore do we not need to make these mistakes once in order to know where this boundary lies?
In a world now that is not so fussed with the idea of commitment, to a job, to a relationship, to a house, anything really. Where change is so frequent if you don't stop and look often enough a whole chapter has passed you by, it is too easy to make these mistakes, and what's worse is its even harder to see where these boundaries are now.
Lets look solely into relationships here where most mistakes lye. Is there the forgivable mistake?
A lot of people will state that if you abuse it you lose it. But is this really a realistic view in today's society?
I agree on the terms that if your partner has been continuously heading over the barrier for months then yes this is not a forgivable mistake as they repeated it and therefore have not learned anything. In this situation, if forgiveness is given then this gives them free reign to walk all over you.

But...

If this mistake was once, and the partner was told by parties concerned. Then more thought needs to be taken into it. They stepped over the boundary that was not yet set and have realised they over stepped it. The rule of trial and error seems to be greatly accepted in all areas except relationships. Do we not need to make a few mistakes to learn and move on to make things better?
Still in this situation I realise there are still some circumstances that are unforgivable, like sleeping with your best friend and similar, but there are situations in which you have to sit back and think, am I ready to throw away something on someone's mistake when im just as vunerable to make one too? Should I be on the moral high ground here, or is that being presumptuous?
I say be more of a realist than romantic in this situation and really consider on whether their betrayal is REALLY something you cant get over before doing the all too familiar cocktail in the face.