Glitch: When you bring up your list of reservations, current day reservations show as "under way", thereby preventing you from cancelling the reservation online.

Workaround: Copy the reservation number from the list of reservations; then paste the reservation number on the left side of the "confirm or cancel page" (the side of the page where you look up individual reservations by the reservation number and your name rather than by your username and password.) Voila, your current day reservation appears as it did before the website was damaged by Marriott's IT folks, and you can cancel without having to call reservations.