By now you've most likely heard about the egregious SSL flaw that has existed in OSX and iOS for a while now.
Yesterday, Apple (finally) released 10.9.2 which addressed the flaw, as well as some other features. Upon upgrading, I was more than slightly frightened to see this error when trying to open github.com:
Had I been compromised already? I didn't see any other reports of this yesterday, so I just closed my laptop and hoped that a reboot fixed it.
On a suggestion from a coworker I reset my keychain and the SSL problem disappeared. To do this, launch Keychain Access, then open Preferences. Click on Reset My Default Keychain.
This is a gigantic pain in the ass, but it does fix the problem. Your old keychain will be backed up and stored in your Keychains folder.
I'm still unsure why the update caused this issue for only some of us, but hopefully this will help those of you who are experienced this issue.