رُفِعَ عَنْ أُمَّتِي الْخَطَأُ وَالنِّسْيَانُ
“Error and forgetfulness have been lifted from my community.” (Wording cited in the textbook)
People plainly still make mistakes and forget things. To interpret the expression intelligibly, something concerning the legal consequence or accountability must be understood as omitted.
The textbook explains the required addition as ay hukma-huma, “that is, their ruling.” This is an example of necessary implication, not a claim that every legal consequence of every mistake disappears in every circumstance.