Question About Registration to Waiting List Incidence

One of our Event locations has had recent problems with editing a booking entry when the event is fully booked. Here is her description and I was wondering if you could shed some light on what you think is the cause:

I have been editing entries without problems for a some years now. There is a facility to edit, delete or enter a new entry. I do it all the time.
What I did was click on the box next to the name and then clicked on the name to edit as prompted. When the booking came up I changed a room share entry from 'anyone' to a person's name. I did not change the event capacity number. The event remained fully booked but the person I edited immediately was put on to the waiting list..

Many thanks

Gayle Atherton