This occurred on an actual event, too, but I created a test event to try to debug with. Once an event has hit its capacity (2 for my test), when a user tries to register, he sees the "event is full" text, along with the form fields to fill out, but the Submit button is disabled (hand cursor over it, but clicking does not fire an action). So the user cannot register and get a spot on the waiting list after all.
Is there a setting somewhere specific to this, to enable the waitlist? Is there a workaround?