Yekke – good points in response to Avi’s claim that there is no punishment for a D’Rabbanan done b’shogeig.
I am not familiar with any of the sources on the topic, but my reasoning would be that there would be no reason for anyone to learn the halachos d’Rabbanan.
Unless, you want to distinguish between someone who never learned the halacha (and could have done so) and a case in which he did know the halacha but erred.
But I find it hard to believe that someone wouldn’t be punished for negligence in a case like the one I brought where she should have looked more carefully at the container. (I’m not saying that I couldn’t have done the same, but it does show a certain lack of carefulness with halacha).