Pesahim 72a and Pesahim 72b, versions of to’eh bidvar mitzvah, when R. Yehoshu’a (according to his simple reading or according to R. Meir) accepts a person’s action as a mistaken thought s/he was performing a mitzvah, and then actually did perform a mitzvah or did an act that could have been a mitzvah that day, thus exempting him/her from the usual hatat for having violated a karet prohibition unwittingly.
