It doesn't happen often but it has happened.
From Wikipedia: The Catholic Encyclopedia notes the historically obscure resignations of Pope Pontian[6] (230–235) and Pope Marcellinus (296–308), the historically postulated resignation of Pope Liberius (352–366),[3] and that one (unspecified) catalogue of popes lists Pope John XVIII as resigning office in 1009 and ending his life as a monk.[7][8] The first historically unquestionable[3] Papal resignation is that of Pope Benedict IX in 1045. In order to rid the Church of the scandalous Benedict, Pope Gregory VI gave Benedict "valuable possessions"[3] to resign the papacy in his favour.[9] Gregory himself resigned in 1046 because the arrangement he had entered into with Benedict was considered simony. A well-known resignation of a Pope is that of Pope Celestine V in 1294. After only five months of pontificate, he issued a solemn decree declaring it permissible for a Pope to resign, and then did so himself. He lived two more years as a hermit and then prisoner of his successor Pope Boniface VIII and was later canonised. Celestine's decree, and that of Boniface concurring, ended any doubt among canonists about the possibility of a valid Papal resignation.[10] Pope Gregory XII (1406–1415) resigned in 1415 in order to end the Western Schism, which had reached the point where there were three claimants to the Papal throne: Roman Pope Gregory XII, Avignon Antipope Benedict XIII, and Pisan Antipope John XXIII. Before resigning he formally convened the already existing Council of Constance and authorized it to elect his successor. There were als several "Conditional" resignations: Before setting out for Paris to crown Napoleon in 1804, Pope Pius VII (1800–1823) signed a document of resignation to take effect if he were imprisoned in France.[3] It has been claimed that during World War II, Pius XII drew up a document with instructions that, if he were kidnapped by the Nazis, he was to be considered to have resigned his office, and that the College of Cardinals were to evacuate to neutral Portugal and elect a successor.[11] In February 1989 Pope John Paul II wrote a letter of resignation to the Dean of the College of Cardinals, which said that he would resign from the papacy in one of two cases: if he had an incurable disease that would prevent him from exercising the apostolic ministry; or in case of a "severe and prolonged impairment" that would have kept him from being the pope.
These are not comparable cases. I guess I should have been more specific for the ignorant. For example, Pope Pontian, resigned because he thought his absence would be bad for the Church. His decision was a disaster, which lead to a schism. Pope John Paul II was in far worse health, but never abandoned his vocation.