Why when I increase SSL certificate security on my mac am I unable to use the app store (preflight error)?
This is a known bug in the App Store. Apple needs to fix it.


  The only problem with forcing a check of revoked certificates is that
  Apple has a flaw in how it validates Mac App Store updates. With the
  option selected as shown in the figure below, you may be unable to
  perform updates through the App Store program. To fix that, launch
  Keychain Access, change the preferences to Best Attempt, update your
  apps, and then reset to the stricter setting. This is something Apple
  should clearly fix, since all the components of this situation are
  under its control.


http://www.macworld.com/article/1162472/keep_your_mac_safe_from_web_security_flaws.html