by forget I think they mean like, when you look at someone, the first thing that comes to your mind is what he did to you, even after 10 years when you see him you will instantly remember that bad thing he did
that is what not forgetting is defined here I think
I would forgive just about anything, and I have many times, you just can't go around hating people, sooner or later you have to forgive them especialy if they are in your direct enviroment(you encounter them at least once a month)
as for forgetting, I would forget anything except lies
you can curse at someone by mistake sometimes, you can hit someone by mistake anything is possible, but a lie is no mistake, this is something malicious and intentionnal
sometimes out of fear I tell a lie by mistake, but I always say the truth after I realise what I am doing
in worst cases you have to say the truth again before the day is done
that is why I do not forget a lie, someone intentionnaly wanted to hurt me to protect themselves that is what a lie is for