Right now there is nothing in the system physically preventing users to overbook events. While we can add notification to Events to pop up if the capacity is reached, these will not pop up until the page is refreshed.

What would be useful is to either have an option on the event details that allows/denies overbooking (also maybe waiting list functionality?), or have fields for "capacity" and "registrant count" added to the "Add registrant" button expression list (making the button Visible in design mode only if capacity>count of registrants).