Roman Catholics, Eastern Orthodox, and Oriental Orthodox are known to venerate and pray to those whom they consider to be saints who have died: Cyril of Alexandria, Basil of Caesarea, etc. These traditions also are known to reject the doctrine of the perseverance of the saints: those who come to be saints at one point in life are thereafter capable of apostatizing, losing the salvation which they once possessed.
This leads to a question: do these traditions have grounds for believing that it is impossible that those whom they regard as saints apostatized on their deathbeds? Raising this hypothetical may be thought of as offensive, but if it is possible or may even have happened, the real offense would be against Christ.
Supposing it is possible that some of those whom these traditions regard as saints apostatized, is it not also possible that members of these traditions are venerating and praying to someone who is actually in hell? Orthopraxy depends on orthodoxy. If we cannot be sure someone is orthodox, should that not have implications for one's praxis involving them? Surely, veneration of and prayer to one who may be in hell cannot be obligated. I expect most of the aforementioned traditions would reject this supposition.
What is the alternative? Infallible assurance of another's faith (let alone perseverance) is ordinarily impossible. If these traditions argue that it is not possible that those whom they regard as saints apostatized - if one can have infallible assurance that those who are venerated and prayed to remained saints unto death - such can only be the case if they accept an extraordinary source of and means for such knowledge. I can think of no other possibility than that it somehow has been divine revealed which saints persevered.
The mechanism by which these traditions believe God has revealed those who persevered currently differs among these traditions. For example, papal approval obviously will not be viewed as necessary in Eastern Orthodoxy and Oriental Orthodoxy. Actually, though, while Roman Catholics would argue that papal approval is now required by canon law, the process of canonizing saints is apparently revisable. In fact, it seems that even Roman Catholics admit that papal involvement was not only unnecessary but also entirely absent in the first nine centuries (link). A Roman Catholic, then, would have to be open to defending different mechanisms.
Another consideration is that Eastern Orthodoxy and Oriental Orthodoxy consist of autocephalous or self-governing churches. This may lead some churches within these broader traditions to venerate as saints those who are not venerated by another church within the same, broader tradition. For example, within the broader tradition of Oriental Orthodoxy, the Ethiopian Orthodox Tewahedo church venerates John of Damascus (link), whereas the Coptic Orthodox church does not.
This is interesting in that John of Damascus opposed miaphysitism, a Christology typically characterized as distinctive to the tradition of Oriental Orthodoxy. In fact, as a Chalcedonian, John of Damascus is ironically anathematized by the Ethiopian Orthodox. Here, the problem is even more obvious than the hypothetical I posed above: how can one coherently venerate or pray to a theologian whom he regards as having been anathematized?
One possibility is that anathematizations can be lifted. One case of this happening occurred when the Roman pope and Eastern Orthodox patriarch of Constantinople mutually agreed to nullify the anathemas of 1054 (link). This appears to be mental gymnastics, however, and I know of some within the Ethiopian Orthodox Tewahedo church who would sooner admit John of Damascus should be removed from their synaxarium than defend the possibility that John of Damascus is venerable or worthy of hearing one's prayers.
Another possibility is to suggest that a synaxarium (or parish, priest, bishop, or any other vehicle of communicating a list of saints) is only infallible indirectly, i.e. insofar as those who are listed as saints have been declared so by that which is the direct mechanism of divine revelation - say, a synod.
Regardless, the logical end of each of the aforementioned traditions is a hyper-realization of solo ecclesia. These traditions might speak of "historical investigation" that goes into their decision-making. These traditions might speak of only "recognizing" who already is or is not a saint. But this cannot be.
Given their rejection of the doctrine of the perseverance of the saints, to rule out deathbed apostasies as a live possibility presupposes that these traditions not only have the prerogative of interpreting divine revelation but also have the prerogative of actively creating such ex nihilo, upon which they further obligate the consciences and practices of their members.
For these traditions, new and public divine revelation is ongoing. They are not merely drawing out inferences from the God-breathed Scriptures but are claiming themselves to be able to draw out the breath of God - through their papal, synodal, or popular voice - at will. Protestations to the contrary will logically entail, as has been pointed out, that members of these traditions could be venerating and praying to someone who is actually in hell.