When, in 1410, Baldassare Cossa (c.1370-1419) was elected Pope John XXIII, there was only one small problem, the church already had a pontiff (Pope Gregory XII). In fact, it had two, as Pedro Martinez de Luna claimed to be Pope Benedict XIII. This was the time of the Western Schism, a split within the Roman Catholic Church, which lasted from 1378 until 1417.
Cossa took the name John in honour of Saint John the Baptist, the relic of one of whose fingers he claimed to possess. He only remained 'pope' for five years; he was deposed in 1415 and tried for a number of crimes.
Baldassare Cossa died in Florence in 1419 and is buried in the Baptistery. The inscription on his tomb (the work of Donatello and Michelozzo) proclaims: 'JOANNES QUONDAM PAPA XXIII'.
On October 28th, 1958, Cardinal Angelo Roncalli was elected pope. He chose the name John XXIII, thus affirming the status of Baldassare Cossa as an antipope.