I've been using Amazon Video On Demand recently to rent digital movies for viewing on my PCs. We've got two PCs in the house that are set up for Amazon's video on demand service (each PC must have the Amazon Unbox video player installed and be registered with your Amazon account; videos will only work on the PC you specify when you order the video). Our family PC in the living room has always played them fine, but I hit a major roadblock when I tried to view the first movie I had downloaded for my personal PC.
Each time I tried to launch the video in the Unbox player on that computer, I was met with a dialog that said: "Unable to play file, please try again later." It also gave an error code: 0x80040217 Also, since videos are registered per-computer, I couldn't copy it to the other PC for viewing. (I tried.)
After a couple of days of trying to get it to work, I finally went to Amazon's Video On Demand Support page and used the Contact Us feature to submit an email. After a couple emails back-and-forth they recommended I contact their phone support (available at the same page), which is where we finally figured out the issue.
For whatever reason, they said that the COPP protection they use on their videos was not working on my PC. The support representative disabled COPP for the problem PC on my account and when I attempted to launch the video again it worked. It was funny the way she described it, because she made it sound like my computer was not "powerful enough" to handle COPP, when in fact my computer is darn near epic in terms of components and performance. I think the real problem might be the fact that I installed a new graphics card a week ago (after I had initially installed the Amazon Unbox software). I bet if I had uninstalled/re-installed Unbox it would have worked, but I'm perfectly happy with them disabling COPP.
Anyway, this information is definitely not in Amazon's knowledge base, and I didn't see the answer anywhere else on the Interbutts, so here's hoping Google picks this up.