In reference to ticket #3429 (originally reported as a bug).
When an agent logs in or out of a queue, the agent number they enter is not validated against a list of valid extensions. For example, you could have extensions 100 and 101 as the only ones defined, but entering 99, 01, or even 101* will be accepted and the agent will be logged into the queue even though the extension does not exist. This will cause a queue to not be empty, even though no valid extensions may actually be logged into the queue (which can cause problems for queues with "leave when empty" or "join when empty" set).
Yes, the REGEX on the queue can be set to "validate" it, but without any built in validation of extensions, you could potentially have to update every queue's REGEX when adding/editing extensions. It would be very helpful to have all invalid input thrown out by default, and then use the REGEX to further refine any screening you want to do.