But I Don't Want To Be Pope!

As it became clear that Pope Paul I was nearing his death in 767, the various political factions of Rome began maneuvering to get their preferred successor elected to replace him.  A powerful noble family from nearby Nepi, lead by a man named Toto and his brother Constantine, sought to stage a military overthrow of the Papal government, naming Toto the Duke of Rome, and forcing the consecration of Constantine as the new Pope.

After Paul's death, the Nepi family set their plans in motion.  They invaded the city, and forced the local clergy to ordain Constantine as a monk, then a subdeacon, then a deacon and finally as the Bishop of Rome, all within the span of a few days (it's amazing how quickly a sword pointed at some select throats can cut through the church's bureaucratic red tape).  Meanwhile, Toto assumed military governorship of the city, and trapped his rivals, Christophorus and his son Sergius, both high-ranking officers of the church, within the Lateran Palace.

Christophorus and Sergius begged Toto to allow them to leave peacefully so that they could retire to exile in a monastery.  Once they receive his permission, however, they instead went to the Lombard king Desiderius, and requested his help in restoring ecclesiastical authority.  Desiderius, hoping to acquire a loyal ally on the Papal throne, agreed, and sent a contingent of troops, led by his priest Waldipert (apparently anything passed for a real name back then) to drive out Toto and Constantine.

The Lombard army defeated Toto's forces, and he was killed in battle.  Soon after, Constantine was arrested and imprisoned.  Waldipert then sought to elevate someone loyal to Desiderius to the Papacy, rather than restore the authority of Christophorus and Sergius.  To this end, he entered the nearby  monastery of St. Vitus, picked out a random lowly priest named Philip, and declared that God and Saint Peter had anointed Philip as the new Pope.  After dressing Philip in the Papal regalia, and having him lead a traditional ceremony they declared him Pope Philip I.

Unfortunately for the Lombards, the people of Rome weren't going to accept a second sham consecration, and Philip's Papacy received little support.  By the end of the day, he was forced to leave the Lateran Palace, and return to his monastery, never to be heard from again.  Christophorus and Sergius returned to power and oversaw the election of Pope Stephen III, who is viewed as the legitimate successor to Paul I.

Antipope Philip, as he is now known, never had any ambitions to the Papacy, and was merely chosen as a candidate of convenience by Waldipert.  He certainly didn't wake up that morning expecting to be Pope, and went to bed that evening Pope no longer.