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. It also affects any USB install disks you've created using the downloaded installer.