When deciding if an errata is public or not, the perl code says "if this errata belongs to any private channels, it's not public". The logic should really be "if this errata belongs to any public channels, it's public". See modules/rhn/RHN/DB/Errata.pm, sub is_public This affects the public errata pages, specifically RHBA-2008-0591, which is in the rhel 5 client channel, and thus viewable in the rhel 5 client errata list, but returns 'not found' when you click on it.
An example of non-accessible erratum: Check the errata available at http://rhn.redhat.com/errata/rhn-tools-errata.html Click on 'KVM Guest management' erratum: http://rhn.redhat.com/errata/RHEA-2009-1639.html yields "Page Not Found".
Fixed in 53ad63dd6619cd539e0c4099617846e7fbbf12f2.
And on WEBDEV.
Verified on webdev on 3/5/10
Addressed in 521