Mac: Make a bootable USB flash drive is the easiest (and fastest) way to get the newest version of OS X installed on multiple machines. However, if you still have one sitting around, TidBits found out that an expired certificate file means they won't work any more. A certificate is required for every app in the Mac App Store and it turns out that all of the various OS X installers had a certificate that expired in February 2016. Pretty much every app affected by this has already been updated already so if you download something now, it won't have any issues. However, if you're like me and like to keep a bootable install disk or USB drive around for troubleshooting, you'll need to download the installer again and remake your USB drive.