Mongoid will never log a message if somebody overwrites an already defined scope.

In file:
https://github.com/mongoid/mongoid/blob/master/lib/mongoid/named_scope.rb#L129

if !scopes[name] && respond_to?(name, true)

has to be changed to

if scopes[name] && respond_to?(name, true)

Also would have an option which allows throwing an exception instead of silent logging (more helpful while development imho), maybe something like safe_scoping: true|false (default: false).