Josh Centers, writing for TidBITS has yet more fallout from the Mac App Store's Certificate fiasco:
File this one under “Obscure problems that could ruin your day.” TidBITS reader Randy Singer reports that due to an expired certificate, OS X installers downloaded prior to 14 February 2016 won’t work.
Here's what this means - if you have a previously downloaded versions of OS X, they will no longer work. I keep several versions of the operating system stored on my Drobo for archival purposes - they've often come in handy when troubleshooting for friends in family. So, I'm now in the process of re-downloading gigabytes of data, again. That is, assuming you're allowed to download the Installers. See, Apple won’t allow a newer Mac to download versions of OS X that aren’t compatible with that Mac.
Likewise, if you've previously created a bootable flash drive to install OS X, that drive may no longer work. You'll need to re-download the installer and re-create that backup drive.
"Hey Siri, add fix bootable flash drive to my Omni Focus List."
...waits while nothing happens...
Remember when Apple stuff just worked?