what are you least likely to forgive?

[Expat]

Another thing I hate is when people start arguing with you, but can't take a response back. I never, ever shut up when somebody argues a point with me, and anybody that can't take it is bound to get very nervous.

Inentionally hurting others
Willful ignorance

I agree with those.

Also, someone talking loads of nonsense about a subject; then, when you object and/or tries to correct them, get offended and/or laugh at you as if you were the one who doesn't know what you're talking about. Such a person has ceased to exist as far as I am concerned.

Lying intentionally.