A new feature in iOS 9 is being able to update your iPhone overnight while it's charging, so you don't have to deal with it during the day. You'll get a pop-up like this:
Last night, I set my iPhone to automatically update overnight, plugged it into my charger, and went to sleep. When I woke up, it was not to my iPhone alarm, but to the sun streaming through my window. I OVERSLEPT FOR WORK.
Turns out, this overnight automatic update takes you to the "Hello" screen you usually see when you start using a new iPhone. You need to click through the whole thing in order to resume normal iPhone functions.
It also turns your fucking alarms off.
I felt pretty sheepish getting to work late (all this for the unicorn emoji), but turns out I'm not the only one who's had this issue.
Let my tale be a lesson to you all: Update your phone during the day, or use a different alarm clock if you go the overnight update route.