Wasted by WordPress 3.8

Blind panic this week when our upgrade from 3.7 to 3.8 left our clients unable to log in. A bit fundamental that. This was on our major domain multisite implementation. Which means we won't be the only ones to be hit. Google is supposed to be your friend but all relevant search terms produced nought. Panic increases. Luckily I can still login as the network admin but what was happening and what do we say to our clients? [...]