Dave Sigafoos:
Is there a listing somewhere of all the pages that don't exist?<G>
Yes! ;-)
The MediaWiki: namespace is special: it's essentially a configuration database, mostly for Wiki messages, but also for style sheets and other stuff -- like JavaScript -- that gets sent to the client.
You can see a listing of all the MediaWiki: messages by going to Special:Allmessages. Each entry is an "article"; if one doesn't exist, a default value is used. Most of them won't exist initially, like Common.js, and hence use their default values -- in this case, blank.
Ian