Google's App Specific Passwords in Calendar.app
At some point, the only way to use Google Calendar with Apple's Calendar.app was to set up two-factor authentication and then create an "app-specific password" to use in the application. On my laptop, running Mavericks, this quit working a couple of days ago. Trying to create and enter a new password didn't work either. Eventually, I used this as an excuse to install El Capitan where the Internet Accounts System Preferences now directs you to authenticate directly via Shibboleth to configure a Google Account. But I suspect this means that Mavericks (and previous) systems will find it hard to work with Google anything going forward and will need to be updated to Yosemite (or El Capitan).
