http://en.wikipedia.org/wiki/Papal_infallibility is not a bad definition.
As it says not everything a Pope says is held to be infallible..... and not everybody who says they are Pope is accepted by the Church as being such - either at the time or later
Go back far enough and the Pope as Bishop of Rome is only the first among equals which would not meet the requirement.
To quote the 'Bull of indiction of the sacred ecumenical and general council of Trent'
Meanwhile our cruel and perpetual enemy, the Turk, attacked Italy with a vast fleet, having taken, ravaged, sacked several cities of Apulia, and carried off a booty of captives; we, amidst the greatest alarm and danger of all, were busied in fortifying our shores....
Now the Pope did not spend every minute in Rome - and nor was Rome a great fortified city at that time so it is not impossible that a C16th Pope could be captured by a raiding party...
As it says not everything a Pope says is held to be infallible..... and not everybody who says they are Pope is accepted by the Church as being such - either at the time or later
Go back far enough and the Pope as Bishop of Rome is only the first among equals which would not meet the requirement.
To quote the 'Bull of indiction of the sacred ecumenical and general council of Trent'
Meanwhile our cruel and perpetual enemy, the Turk, attacked Italy with a vast fleet, having taken, ravaged, sacked several cities of Apulia, and carried off a booty of captives; we, amidst the greatest alarm and danger of all, were busied in fortifying our shores....
Now the Pope did not spend every minute in Rome - and nor was Rome a great fortified city at that time so it is not impossible that a C16th Pope could be captured by a raiding party...