I spent the better part of two days attempting to get two different XBox Ones to download the initial 500mb update at my house. I had been trying both wired and wireless connections. They were failing every time at 74% (127mb remaining). I found dozens of posts online where people were having similar issues but saw no resolutions. After chatting with 3 Microsoft techs and one person on the phone, I was able to trace the issue back to my network setup. Although my internet connection was performing at a solid 30mbps download speed, whenever the Xbox plugged in to a port on my router, it would fail. Once I plugged the Xbox direct to the cable modem and reset the modem, the update completed successfully.
I would suspect the Xbox has an extremely short timeout set if a connection hiccups but I could not get a solid confirmation from Microsoft on this. Hopefully this helps someone on Christmas morning.