Pragamatically, I would say that the answer is yes, because human beings inevitably make mistakes. However, I think the tendency in formation is - at least initially - to err on the side of generosity, and thus give people a chance so as to see how it works out. So I don’t think that such errors are common, and for someone who truly has a vocation to consecrated life or priesthood, there will always be another chance in the future to find their path if they remain open to God’s prompting.
But what is more common, sadly, is someone leaving formation, or not being admitted in the first instance, and being unable or unwilling to listen to the reasons why they are not invited to progress. I’ve met a number of people who are - understandably - angry or upset about being rejected (as they see it) and who protest that they were not given adequate reasons for having to leave. However, in conversation it becomes apparent that they were indeed given good reasons, but are simply unable to accept or even hear them, because it can be painful to admit to ourselves that we do not have a vocation to the state of life to which we aspire. Thus the reasons for one’s inability to progress are turned outwards, and transformed into assumptions of persecution by formators, or their inability or unwillingness to understand the individual who has been asked to leave, or the perceived genuineness of their call.
To me this is always a red flag; if someone basically argues that their formator or community was against them from the start, or never gave them a chance, then they really weren’t called to community life, because such decisions are never the product of one person alone, but are made with multiple (name removed by moderator)uts from many skilled people. But refusing to accept that judgements were made in good faith rather than from personal antipathy protects the individual from having to examine themselves too closely, or acknowledging that their departure is the will of a group who have no vested interest in misrepresenting their assessment of the person departing.
So I would say that yes, formators can make mistakes in being either too inviting or too cautious; but that usually a dissonance between formators and formed stems from the latter party, who by definition is not as skilled or as aware of the issues of formation as are their directors. That’s a difficult truth to accept: that we know less about our suitability for religious life and priesthood than do the people who are guiding us, but it’s what the church calls us to accept inasmuch as no vocation can proceed without the permission of the proper authority.
…just came back from a silent retreat at mount carmel in niagara falls ontario with the ocds, and it was beautiful and quiet for much reflection…
I have yet to get confirmation whether it’s a “yes” or “no”, so I’m already prepared for any answer. I was initially very disappointed. I talked to my parish priest about it, and he told me to take it as a “grace from God”. So that’s what I’m going to do. 
My heart is all for Jesus. As St Therese of Lisieux would say, “Nothing can charm me here below”.