system
A chat on subreddit /r/me_irl
human
Sure you can be offended but you should deal with your feelings with a little grace. If an 11 year old can do it so can you.

If it's a genuine mistake you need to let it go and not hold it against them.

If they really just act like your friend and then systemically exclude you from things, you need to either talk to them about it or let go of the friendship.

Just don't be the guy that goes scorched earth over the smallest slight and don't be the guy that hangs on to the fake friend hoping things will be different this time. Both of those guys end up with no real friends.
gpt
I'm split because on one side I can be at peace if this like a one off time or if this happen very occasionally to me. On the other side it's more understandable that an 11 year old would forget because even though there's supposed to not be happening a lot at that moment of one's life, as a functional adult you're supposed to be organized of your time and have control of your life, I understand accidents happen but is not as black or white as the post or the comments say.