One question is whether or not canonizations done by the Roman Pontiff are infallible. But this question can hardly be answered unless we first know what is being asserted by a canonization. If the Church wishes to call certain persons “Saint”, that is Her prerogative. But what does it mean?
Many commentators claim it only means that the person is in Heaven. No, I don’t think so. The “least in the kingdom of heaven” are not Saints. Merely being in Heaven can occur by a person who lives a very sinful life, repents on his death bed, and receives Last Rites devoutly. Then if they receive a plenary indulgence at the hour of death, they might go straight to Heaven (which of course means after the particular judgment).
The reason that the Church canonizes Saints is so that the faithful will have examples of how to apply the Gospel in many different ways in many different states of life and situations. The Saints are held up to the faithful to be imitated, and to listen to their teachings and insights into the Gospel.
So if a canonization is asserting a truth, it is the truth that the Saint imitated Christ well. This implies that they will have Heaven as their final destination, but it does not rule out a period of time in Purgatory.
What if a Saint is very holy, and is living a contemplative life. But God wants that Saint to take up a new task, in the active life, so as to help more souls. The Saint might sin more, venially, in the active life, due to his frequent interactions with sinful humanity. And this could mean time spent in Purgatory after he dies; whereas, if he had stayed in the contemplative life, he might have spent no time in Purgatory. But he did the will of God, and he went to battle with sin in order to save sinners from Hell.
A Saint might make a mistake at the end of their life, meaning they would have to spend time in Purgatory. Consider how holy Moses was, but he sinned not long before reaching the Holy Land (when he struck the Rock, while doubting, and water flowed out). So he was not permitted to enter the Holy Land. This can be read as a figure for those who sin near the end of their life, and so cannot enter Heaven directly, but only through Purgatory, despite a holy life.
Solomon was holy, until the end of his life, when he went astray due to his power and riches and many wives. His success was his undoing. If he went to Heaven at all, he must have spent a long time in Purgatory, before entering the Limbo of the fathers.
So canonization does not mean merely that they make it to Heaven at some point, that is too little a meaning; it does not mean they went direct to Heaven after death, as a person could still be a Saint and spend time in Purgatory. The Church has never taught that the Saints spent no time in Purgatory. St. Therese talks about what she would do if she had to go to Purgatory before Heaven. What canonization means is that the person imitated Christ well.
Edited to Add
So are canonizations infallible or fallible? The question is whether the Church could err and canonize someone who did not imitate Christ will in their life. But there is a third possibility: non-infallible. If canonizations are non-infallible then the Church could not err to the extent of canonizing someone who was wicked or someone who, if the faithful were to imitate them, their path of salvation would be gravely harmed. A person of only ordinary holiness might mistakenly be canonized, if canonizations are non-infallible, but not a person of wicked life.
I do not have a position on this topic. I used to argue that canonizations were fallible, and could err to any extent. Now I think that canonizations are either infallible or non-infallible. Between the two choices, I am leaning towards infallible.