I stayed at a Marriott timeshare resort (Marriott's Grande Vista in Orlando), where two rooms shared a vestibule.

My keycard unlocked the door from the outside.

The vestibule had two inner doors, one leading to the one-bedroom master suite unit, and the other leading to the efficiency unit. If one family had rented both units from Marriott, it would have a been a full two-bedroom condo. But that wasn't the case during this stay. Each room was rented out separately.

Once in the vestibule, my keycard unlocked my unit.

Someone else stayed in the other unit. They had a keycard that unlocked the vestibule door and the door to their unit.

Not surprisingly, my inner door had a security latch that could be locked from the inside, as one would expect at any hotel these days.

Here's what surprised me: The door from the outside to the vestibule also had a security latch from the inside. It would be very easy to latch that security latch absentmindedly, preventing the other party from getting into the vestibule and thus preventing them from getting into their unit. Fortunately, that didn't happen while I stayed there.

I imagine, however, that it happens from time to time. I don't know if the front desk then calls the occupied room to unlatch the vestibule door, or if Loss Prevention would come over to override the latch somehow.

It strikes me that a vestibule serving multiple rooms, whether at a Marriott Vacation Club, a Residence Inn, or any other brand, is an invitation for such problems to occur.

I'm not saying the OP of this thread absentmindedly closed and locked the outer door (or even that the configuration and lock mechanisms were the same as where I stayed). Somehow, it became closed and could not be unlocked by the guests of the adjacent room. Ideally, that should not be possible.