Under what conditions is this a problem? Any user? Any non-admin user? When only one admin user is present? Please provide reproducer steps.

At least one admin is required to retain administrative level access, else recovery methods are required to regain access.

I have one admin and when I edit users in the UI, it errors due to that line preventing any modification of users.

Reproducer:

1. create a non-admin user via the API (or another means, not the UI)
2. ensure the admin field is nil, not false:

2.0.0-p353 :001 > User.find_by_login("wraptest5").admin?
 => false 
2.0.0-p353 :002 > User.find_by_login("wraptest5").admin
 => nil 

3. as an admin, edit the non-admin user

This means "admin_changed?" results in true as it moves from nil to false, rather than false to true as the expression was intended to catch.

[1] pry(#<User>)> changes
=> {"admin"=>[nil, false], ...

Applied in changeset b8391adc2f720734551c57af96a2a50b9699fb58.