Ahh, but Wikipedia also notes that there were other abdications as well, notably John XVIII for apparently no reason, and a botched attempt by Benedict IX, who however actually sold the office. Gregory XII was forced to resign, but it seems like he was pulling some really shady tricks and deserved it.
Fascinating stuff. I happened to remember about Celestine V only because of Dante's scathing condemnation of him in the Divine Comedy, put amongst the Uncommitted in Inferno III and described as 'colui che per viltą fece il gran rifiuto' ('[the one] who made from cowardice the great refusal'). He saw him as
directly responsible for the election of
Boniface VIII.
An unrelated tidbit: this morning, when Benedict XVI delivered in latin his statement about resignation to the members of the consistory, they immediately assumed they were getting it wrong, since it was that unthinkable to them; they had to wait for the translation in their respective languages to be sure about what they heard.
-snip-
As far as I know, the Prophecy of the Popes was a sixteenth century forgery manufactured to comment on the back then contemporary popes, or to favor the election of cardinal Simoncelli, as Wikipedia says. The motti stop describing accurately the coat of arms of each pope after 1595, and it contains several biographical misconceptions and outright mistakes based off the chronicles of historians contemporary of Arnold de Wyon (I wish I could find some source in English, but alas).
Here in Rome so far there's no fear mongering about the "destruction of the city of the seven hills", and I hope it will stay that way