Quote me where the magisterium asserts that canonizations are liable to error.
Can't click on one little link? It's the very first quote.
For the Holy Spirit was promised to the successors of Peter NOT so that they might, by his revelation, make known some new doctrine, but that, by his assistance, they might religiously guard and faithfully expound the revelation or deposit of faith transmitted by the apostles.
Not contained in the deposit of faith? Not protected by God the Holy Ghost.
Gladius, many popes likely believed they were infallible when they decreed a canonization, but post Vatican, this belief is heretical, as it contradicts the Council on infallibility.