Registration on Calendar
Users cannot cancel their registration if they are on the waiting list. Therefore, if they choose another event and don't want to be on the waiting list anymore, it could prevent others from registering because there is a waiting list.
Amy Burgin commented
FYI, users can remove event registration by going to the calendar and clicking the "My Events" button. The list of registered events appears and they can click cancel for each one. The user may need to be logged in to have access to registered events.