I've noticed on a couple of reservations that were canceled well within the cancellation deadline, that charges were still made as if the stay was made. In one case, Marriott cannot find the cancellation number that is listed in the cancellation email, and the original confirmation number has been reissued to another reservation for someone else. The reservation was cancelled 11 days in advance, and was not an advance purchase or deposit reservation. So, I have to wait 7 business days for someone from Marriott to contact me (or just dispute the charge with the credit card company).
So, pay attention to all things Marriott, especially any upcoming reservations for peak/off-peak changes!