According to Dan Morrill, a Google employee working on Android, when you hit the Check now button in your updates, it will check in with the server. If it is denied, it will make you wait until the next batch of updates is released. Mashing the button will do absolutely nothing, not until the next batch of updates rolls out. So if youre obsessing calm down a little. According to Dan, the updates are first sent out to 1 percent of devices. If it goes smoothly, it is sent to 25, 50, and 100 percent of users over what he says is a week or two.
Source: Reddit
If your device is showing a build number of KRT16O, you need to get KRT16S installed. That's the latest for these devices.
According to the changelog, KRT16S fixes several bugs related to APN settings as well as a more serious bug that Google engineer Dan Morrill called a “stop-ship bug”. According to developer Al Sutton, the critical bug relates to encryption and “could have left encrypted devices undecryptable if the user entered the wrong password after an update”.