Actually I discovered this same situation when booking a room at a RI for an employee of mine. We couldn't get him in the Marriott so we booked him in the RI. The rates came up and I clicked on Reserve this Room button. Then I decided to check elsewhere, came back and the room was no longer available. After about an hour it was available again.

By clicking on Reserve this Room I think that Marriott's reservation application holds that room for a set period of time so others can't come in and book it out from under you while you complete your booking; whether your delay is due to contemplating your decision, checking air fares, booking airfares at the same time, etc. This is actually a nice feature. I hate web sites that after you have gone through all the steps they come up at the very end and say that room is no longer available.

Normally when situations like this happen there are additional rooms available for the same price or award availability so we never notice this phenomenon. However, when it is the last one available, we occasionally do. This is also a nice "feature" in that when it does happen you know that this is the last room available at that rate.

Now, it does sound like their TBR system could use some tweaks with respect to if you are in the same session then by going back and changing dates for the same property, the previous hold should be released. Now this is probably NOT a simple change and their current implementation works well the vast majority of the time because there are other rooms available. The cost/benefit ratio probably doesn’t support the effort involved.