Code review comment for lp:~benji/launchpad/bug-636193

517 + # If we didn't find at least one matching handler, then the requested
518 + # scope is unknown and we want to alert the caller that they did
519 + # something wrong.
520 + if not found_a_handler:
521 + raise LookupError('Unknown scope: %r. This can result from a '
522 + 'typo or perhaps you need to create a new scope handler.'
523 + % (scope_name,))
524

This is a bug: its an operational requirement that the server not crash if a scope goes away during a deploy. Likewise we need to be able to add a rule with scope that isn't used yet but will become active on the next deploy. We can (and should) warn about unrecognised scopes in the +feature-rules page but thats a different discussion.