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.