After a couple of days of thinking about it and reading your posts and suggestions (which really got me thinking, which is a good thing!)
- and talking to DD and seeing how she reacted and everything I got to the conclusion that it was the right thing to do. Obviously everybody has his own parenting strategies, and they´ll change over time as well.
In general, we try to teach our children to be able to make their own decisions and make their own mistakes, which would come into the "natural consequences line", wouldn´t it.
In the situation I described I had to decide if I take over her decision and question her ability to choose for herself or if I let her face the consequences. I decided to take her own responsibility in a not so big decision, where she would not get hurt by a mistake at all.
It was not even the slice of bread, at the end of the day, the breakfast box has the same kind of bread in it, anyway, only with salami and together with a couple of goodies that she missed now.
We talked about it in the evening, how someone grew the wheat for the bread and had to work on it and carried it home and somebody else made a bread of it (which is mommy often times - not this time thought) and it is not very respectful to just throw it out. I think she understood what I was talking about. Not that I think that she will never make a mistake again, far from it, but I honestly think that the experience brought us closer, me understanding her better, she understanding me better.
And maybe there is a bit of overinterpretation in here - but it´s fine. I think, it was a good teaching point for us. She wasn´t hurt, and she was surely not forced to eat anything that she totally did not like, maybe it was not as nice as it would have been but anyway.
I agree with amberskyfire, she needs to learn that ones has to finish his/her meal, and we (parents) do this as well, I personally love leftovers (leftover bread fried in butter with a little cheese on it ... mmmh) and I don´t throw out slices of bread or anything like that.
But again, I really loved to see your insights on that point!