User and site-wide default notification send intervals can get "orphaned"

I came across an interesting bug which occurs in both the D5 and D6 versions of the module. In D5 it's particularly bad because it can cause users to unknowingly set their default send interval to "never" and thereby not actually be subscribed to posts they intended to subscribe to...

The bug occurs when you configure the possible send intervals at ?q=admin/messaging/notifications/intervals. You have total freedom to change/remove intervals, etc., and this includes removing or changing values that may have already been saved elsewhere in the database. Example: You once had a "once per hour" send interval which some users saved as their personal preference on their account page, leading '3600' to be stored with their user account. Later, you go and delete or edit that send interval, so '3600' is no longer an option. When this happens, the select box for the default notification interval on the user edit page or on ?q=admin/messaging/notifications will not get its default value of '3600' correctly set, so it will default to the first selection. If the user then saves their "my account" page, this incorrect selection will (unknowingly, unless they are paying very careful attention) get saved to the database.

In D5 it's especially bad because "Never" is the first send interval in the list, so users wind up accidentally configuring their account to never actually be able to subscribe to posts.

I'm not entirely sure the best way to fix this bug, unfortunately - it's a tricky problem... Possibly there should be code that catches this possibility and warns the user that their chosen send interval is no longer allowed and they have to choose a new one? (Perhaps also a warning message on the administrator screen so that site admins know about this possibility and are careful editing the allowed intervals when the module is already being used.)