Did anyone else catch this article from Arstechnica stating an OSX installer cert had expired and will not work?
Here is the crux:
On February 14, something called the Apple Worldwide Developer Relations Intermediate Certificate expired. This sort of thing is typically routine; it was renewed and
developers were advised to update their certificates so their Apple Wallet Passes, Mac apps, extensions, Safari push notifications, and App Store submissions would continue working properly.
There's one edge case for people who frequently troubleshoot and fix Macs, as
pointed out by TidBits: old OS X installers downloaded from the Mac App Store before the certificate's expiration date will no longer work. This includes not just installers for El Capitan, but also downloaded installers for Yosemite, Mavericks, Mountain
Lion, and Lion—every OS X installer issued using the Mac App Store.