And again, third time perhaps the charm. Saw this little gem of a quote and I couldn't agree more.

Acyd Burn:

@bantu: a user must always have a default group.

me:

The (current in SVN) behavior I documented shows that when removed from a group that was set as the default group the user is automatically defaulted to the remaining pre-defined group (i.e. Registered users) membership. Any other user defined group memberships are ignored when determining a default fall back. If there is no pre-defined group membership to default back to, say you created user defined groups and removed the user from the pre-defined system groups entirely, then the group rank and group color are not removed which makes it appear broken, but only because there is no default group to determine those attributes (color, rank, avatar) The same behavior occurs when the user is a member of only one user defined group and no other, upon removal from the group with no other group memberships, the color, rank & avatar are retained even though they are no longer in the group. That is an entirely different problem though, as I do not believe you should be allowed to remove a user from all groups because it creates an orphan situation.

At a minimum all users should be required to retain at least one group membership that cannot be removed and it should be able to be pre-defined or user defined to allow for a default fallback. This would allow for whatever group attributes are assigned (even if there are none defined) to the remaining group membership to be applied to the user.

http://www.phpbb.com/bugs/phpbb3/49415#post174995

Summary: By designing the software to not allow removal of a user from all groups thus creating an orphan situation, the admin would be "nudged" into adding the user to a new group before allowing removal from the previous (and last remaining) group membership. User defined groups should also not be ignored when a default fallback group is needed, if a user only has two group memberships both being user defined, then removal from one should lead to the remaining group becoming the default. That is currently not the case.