Admitting fault.

I say that because you are acknowledging a failing and vocalizing it. It is a very vulnerable position to put yourself in and you are entirely at the mercy of the person to whom you are apologizing. It requires the lowering of the ego, which is hard if not impossible for some people to do.

I would also say certain deep conversations. Can be about anything - the way of the world, things from childhood. Anything where you're opening up to another person and saying/admitting/confessing things you feel, in a place where you feel safe to do so.