 In the sixth series story "Legion", Lister has his appendix removed,  despite mention of his already having had it removed in series twos  "Thanks for the Memory". This was later "explained" in  Doug Naylor (I) (qv)s solo Red Dwarf novel "Last Human" which  said that, due to a freak of nature, Lister had two appendices;  however, because the show and the novels are based in two separate  continuities, it doesnt serve as an explanation within the show  itself.