The view 'Logon' or its master could not be found frustration

Ran into this one again today:

The view 'Logon' or its master could not be found. The following locations were searched:
~/Views/Account/Logon.aspx
~/Views/Account/Logon.ascx
~/Views/Shared/Logon.aspx
~/Views/Shared/Logon.ascx

of course all the files were present because it was working 5 minutes ago (and the 2 weeks or so before that).

I had been using the ASP.Net MVC source project to debug a few things, but reverted to the GAC dll before checking in so that the other devs didn't end up with some phantom project reference. Hence I thought I had not reverted my web.config (and ~/Views/web.config) changes properly.

Anyway half an hour later and a lot of head scratching later I thought I'd see if setting the System.Web.MVC reference to "Copy Local =True" would make any difference
...
it did.

End of story.