There have been numerous reports lately at various help pages on en about encountering pages that mysteriously don't exist (latest
at Village Pump (misc), see URL attached). Various folks have suggesting adding "?action=purge" to the page request URL which
does indeed seem to "fix"the issue instance by instance. However, this has happened often enough that there's clearly some sort of
bug in the caching or DB lookup or something causing pages that do exist to not be found.