Wyndham has a long-standing policy that if you have overlapping reservations (two or more ressies in the same timeframe) one of those must have a Guest Confirmation...even IF both reservations are in the owner's name.
If an overlap (or simultaneous) reservation without a Guest Confirmation is detected by the Wyndham computers within 15 days of checkin, they can cancel both reservations. If you have multiple owners (my two adult daughters and I are the co-owners of our account), and reservations are in different owners' names, you're okay.
There is a situation being discussed right now over on the TUG Wyndham board where an owner forgot to put the second reservation in his wife's (co-owner) name and got canceled. So it can happen.
For readers here who are renting, I think it is unlikely that you would have that kind of problem.
Most of the people who rent have VIP status and have numerous free Guest Confirmations each year. Plus they rent a lot, know the rules, and are very unlikely to make a mistake as flagrant as forgetting to get a Guest Confirmation.
If I were renting, I would not get over-stressed about it, but I would check the reservation and be sure it was in my name.