In my case the booking didn't go through properly on the accor site.

I was booking a room, and at the final screen after inputting my card details, I got a blank page or an error message (I tried multiple times). After 4 times and receiving messages such as "you have just booked the same hotel for the same night" I decided to call the customer services, they transferred me to the hotel who could see 2 bookings (and even managed to change the room type for one of them). I needed 2 rooms, but I was supposed to book one and a college the other, when I saw this I told them not to book their room. As the hotel could see that the bookings were confirmed, I didn't ask for the reservation number (this was a big problem later on).

We arrived at the hotel a few days later (booked on the Tuesday for the Friday) and the reception had no sign of our bookings. What had happened is that the rooms had been reserved, but the payment wasn't authorised, so when I called they could see that everything was ok, but the computer cancelled the bookings a few hours later because they were not able to take the money. I didn't receive an electronic confirmation of the booking, so I didn't receive a cancellation either.
At the hotel we were told that the booking was not accepted because there were no funds on the card we used, and they told my partner that she wasn't allowed to work in the hotel lobby and had to leave.

After a loong chat with CS and the hotel, they accepted to reinstate our bookings as they had been booked, but only for goodwill because we are such valued customers...

So as you can see the issue was when they tried to take the payment automatically. I guess that it is different for non pre-paid bookings.