I started seeing an issue in Chrome (38 on OSX 10.7.5) accessing GitHub. I got a Privacy error. One of the recommendations on SuperUser, and the reply I got from GitHub support, was to delete all expired certificates in Keychain Access, close the browser, and reboot the machine. I followed the advice.
Now, I cannot access many other sites: LinkedIn, StackExchange sites etc. - all with the same error. I'm forced to use another browser (which btw, shows an error next to the HTTPS lock icon). The Keychain Access certificates view does not repopulate. I might have deleted something I shouldn't, but shouldn't new certificates be downloaded as needed? What do I need to do to gain access to all the HTTPS sites again?