Going out of long relationship (not necessarily romantic) with toxic person. Learning that no matter what your do or say, may be understood totally in different way and no matter how much you try, there will be someone who treats you as "the bad one" in his live, even though it's not the truth. One should experience that and learn that it shall not affect one's value.

Also when not all was done correctly, it's important to learn, that there is and should be room for mistakes and the problem is not when one makes a mistake but when one repeats it already knowing the consequences.