:dancing-banana:
Sometimes a penance includes a good deed that can't be done in a pew or at the altar rail. Sometimes it can't be done right away or all at once. ie. Making restitution, a novena, a mid-week visit to the Blessed Sacrament.... I'd advise the forgetful one to simply do the penance, or if he doesn't recall what it was, to do SOME type of penance, then to relate the entire matter to the priest. Even if is 100% laziness, confess this as well!