I've been having issues with Zappos for a couple days. I called up support yesterday and they said they were "upgrading the website and had bugs they were trying to get fixed." Not sure if this is related or just a coincidence.
Probably coincidence. Companies that expect a lot of Christmas traffic minimize changes from Thanksgiving to Christmas, and web retail gets more traffic during business hours in America, so I expect that this weekend and last weekend saw a lot of code deployments, and so things are more likely to be broken specifically right now than pretty much any other time of the year.