good intentions

To what extent does having good intentions absolve one from culpability for the consequences?
I've found myself on different sides of this question.
A couple years ago, i made a comment to a lover that ended our relationship. She was telling me about how she'd lived through hardships, and i said that i had seen that in her face when we first met. Not just in her eyes, but in her skin, she had the look of someone who had survived alcoholism, or some such. One of my friends noticed it too. A couple weeks later, she said it was the most hurtful thing anyone had ever told her. I was stunned, and tried to assure her that my intentions had been nothing but gentle and loving. I was just agreeing with a point she was making. This was a woman who had experienced far more than her share of positive reinforcement in her life, concerning her looks. You might think that would have made her secure in her beauty...or perhaps you're not surprised it made her profoundly insecure in the other direction. I don't think my comment was the reason the relationship ended, though. The imbalances between us had already been there, and maybe she subconsciously needed something to grasp on whereby SHE could end the romance, thereby avoiding the possibility of rejection, a demon she had long grappled with.
Anyway...
There's a part of me that has long felt that intention is more important than effect. An understandable reaction to living in this world, where people generally take wayyyy too many things personally...because this is a society which fosters insecurity, rather than its opposite. Trying to anticipate the million and three ways that anyone might take something the wrong way is such a daunting task, that very often the only sensible thing to do is tend your own garden, be pure with your own intent. On the other side, of the 100 things the average person will take personally this week (or this day, or this hour), 99 of them should be ignored. Didn't get that job? Mom never liked you best? The popcorn server gave your friend an extra squirt of imitation buttery goodness, but not you? Do not take ANY OF THAT personally. It ain't about you.
Except that 100th thing. Get your shit together on that, willya?
But intention isn't everything, and it took a recent interaction with my Mom to remind me. She generally loves me, and wants happiness for me. But she's stuck in a rut where she wants HER definition of happiness for me, not mine. She frequently hammers me about getting published. She does it in a fairly gentle way, and with the best of intentions. But she knows that my definition of success is different from hers. At what point do her good intentions become irrelevant? At what point does she need to take responsibility for behavior which is controlling and hurtful? It's doubly tough to understand, when the mother i knew as a youth was so loving and accepting. But she's becoming another version of my father. I have to hope that somehow i'll be able to get through to her.
So in what way was my comment to my girlfriend (hurtful with the best of intentions) different from my mother's words (hurtful with the best of intentions)?
There's no one single answer, of course. But good intentions stop absolving us from our actions the moment we know or suspect we might hurt someone. Maybe sometimes even sooner.
Not that hurting someone is always wrong, of course.
Nobody said any of this was easy.