Yes it's rare because of several reasons:
1. The Board of Directors is authorized to approve or deny every potential buyer.
2. Every buyer that is approved to live in an apartment must obey all co-op rules. One of the rules states "no subletting allowed", If a co-op owner (shareholder) violates this rule, they are subject to a penalty and the Board of Directors has the authority to remove the tenant.
3. If the co-op owner (shareholder) chooses to sublet without notifying the Board of Directors, most likely the neighbors will report the tenant to the Board of Directors very quickly. Yes, the neighbors will notice you because they know the owner and if they see somebody else living in the apartment, it's very easy to notify the Board about it.
My advice - DON't DO IT!! If you need to rent or buy - do it the legal way and use a realtor.