Marriott Rewards (including Ritz-Carlton) - Book now, ask questions later!

I just had an interesting thing happen. I was booking a room for next week and debating the merits of the various Marriotts in the area. I used a combination of Google maps and the Marriott listing to find the closest location / least expensive sweet point. I had opened each hotel in its own Firefox tab so I could check the rate for my dates. I finally decided to go with a reasonable $130/night rate at a hotel a bit further away. When I went to book, it told me that rate was no longer available and jumped up to $200/night.

This all took place within a 30 minute window. Furthermore, this has happened on at least two other occasions this year.

So, the lesson learned: book the room as soon as you see something you might like, then go back and change it if you find something better.