I worked at a large hotel in Massachusetts for approx 8 years. Most of that time was working at the front desk. And let me just say, Hotels lie!!! They tell customers many many lies!
No you cannot stay, we are booked. Of course they are not going to tell you that if you don't check out and just stay in the room the hotel can't kick you out!
Of course they are going to say people extened their stays by not checking out. If they told you they overbooked then it would be their fault!
In Mass. a hotel cannot kick out a guest who does not check out. I have had it happen many times, yes you can tell them they can't stay and try whatever you can but ultimately you cannot kick them out!
Yes hotels do overbook, all the time!!!! This is a very common practice, as people said there are always no-shows, it is better for the hotel to overbook and walk an occasional person than not overbook and have no-shows and empty rooms! It costs hotels money to have empty rooms!
A confirmation number does not mean that you are guaranteed to have a room at THAT hotel. It means you are guaranteed to have a room at A hotel! If the hotel runs out of rooms and you come to check in, they will WALK you to another hotel. They will give your your money back and pay for your night at another hotel.