What happened? What became of them? Why did they choose to throw in the towel?
One that I can think of on hand was Pope Celestine V in 1294, one of only three Popes to resign, and the first to do so on his own initiative.
He held the office for only five months before resigning, stating that he wanted to return to his old humble lifestyle as a monk. That didn't happen, as his successor, Boniface VIII ordered him imprisoned, where he died 2 years later in captivity, as Boniface saw him as a potential threat to his legitimacy, or that supporters of Celestine might install him back into office as an antipope.