Users are presented with the following error when attempting a Software Update:
An Error has occurred
The certificate for this server is invalid. You might be connecting to a server that is pretending to be "swscan.apple.com" which could put you confidential information at risk.
Swscan.apple.com is a server used by Apple by for Software Update. A certificate lookup indicates that it expired on May 24th at 23:59:59 GMT.
Presumably, the issue should be fixed shortly; however, for now you can manually download updates from the link below...
Read More [via MacRumors]