I'm having the same issue,

I haven't changed a thing on it, Just got a new nexus 7 32GB yesterday, was having 4.1.2 out of the box, got an update for 4.2,
the update downloads, device reboots, get the android icon showing that it is booting and after a few seconds , get another
android icon, this time flat with a red triangle for almost a minute. The device boots but there is no upgrade.

Now, I'm a total noob and as I said I haven't flashed/rooted or changed anything it was fresh out of the box.

Any suggestions on what needs to be done ?

Appreciate your help.