I've had this problem a lot recently
Never had any issues with my own Macs (until this week), but in my classroom of 17, there are 3 or 4 who have been reporting various issues with update downloads - cannot be verified, may have been tampered with, could not be saved, etc.. Have used combo update on a couple and have also experienced the black screen of nothingness and been forced to reboot.
Had to do this last with my new MacBook Pro only this week.