When trying to load any Unity web player project, if I set an the environment variable "UNITY_DISABLE_PLUGIN_UPDATES" to "yes" it prevents the web player from loading and causes an error message that says "Failed to copy Unity dll.

Steps to repro:
1. open an elevated command prompt in Windows 8.1 (possibly other versions as well) and type:
setx UNTIY_DISABLE_PLUGIN_UPDATES yes /m

2. Then restart whichever browser you are using (User was able to duplicate the problem in IE 11, Chrome 36, and Firefox 30)

3. http://tablet.imaginelearning.com or http://unity3d.com/showcase/live-demos#angrybots or any other web player content (as far as I can tell. I tried 5 or 6 other projects that all showed the same error). As soon as I revert that change by setting the variable to "no" or deleting it entirely and restart my browser, the content works again.