I can’t believe I didn’t think about this during the recent certificate issues with the Mac App Store, but here’s TidBITS’ Josh Centers:
The Apple Worldwide Developer Relations Intermediate Certificate is required for all apps in the Mac App Store, including OS X installers. When used to sign an app, the certificate enables OS X to confirm that the app has not been corrupted or modified by an attacker. This certificate expired on 14 February 2016, causing error dialogs and preventing some apps from launching. Most apps affected have already been updated with the new certificate. But if you downloaded an OS X installer in case of trouble, you may be in for a surprise the next time you try to use it.
To get around this, you can set the system clock back in time, or download new installers. I learned today that you can’t download an older version of OS X than your machine will run. For example, my current-gen MacBook Pro refuses to download OS X Mountain Lion becuase it can’t install it.
One of my Mac minis can run it, so I used it to download the OS, but got a concerning warning from the Mac App Store about downloading an older version of OS X then I’m actually running.
Head. Into. Desk.