We stumble and fall, NOT when we disobey regarding good deeds, or right belief, but when we disobey the word of grace.
We understand on an intellectual level that if we have tasted that the Lord is good (gracious) it will undo shame, but we find in practice that shame persists.
Peter points out this very phenomenon when he says the very goodness (grace) that undoes shame is also the stone that makes men stumble and the rock that makes them fall.
We stumble and fall, not when we disobey the word regarding good deeds, or believing rightly, not when we violate quid-pro-quo, but when we disobey the word of grace.
This caution against disobedience to grace is throughout scripture from the beginning. In the Eden story, Eve disobeys the word of grace when she believes the lie that some action is needed to access God. She believes that she should know good from evil, light from darkness, rather than resting in the paradise of grace, and when she eats the fruit of this lie, she falls from grace and into shame.