my cat seems to forget once in a while where she needs to go. I will never forget the time my mom called me (she lives at their house) complaining about my cat walking into the middle of the living room and taking a crap while my mom was trying to watch Grey’s Anatomy. She was pissed.
Like other people said, use an enzyme cleaner like Nature’s Miracle, and be persistent about putting some in the litter box and making them see/smell it there. Changing litter, location of the litter box, even the litter box itself causes the little furballs to protest and poop elsewhere