I spent the entire morning tying to debug an issue of why my SharePoint site kept prompting my user for their credentials even after they had entered it. What made the issue confusing was this didn't happen for domain admins. To test that theory I added my account to the domain admin group in AD and it worked. Of course I can't leave it like that. A colleague of said that it was prompting me because the account was trying to access resources it didn't have permissions to. Permissions on the physical disk of the server not in the database. So I broke out Fiddler and sure enough the account was getting 401 errors when accessing core.css. So I go look at the permissions of that file and I see that domain admins have rights but that is it. There is the problem. So I gave the Authenticated Users group read access to the file and the problem was fixed. Now why did this happen. Well the title of this post should give you a hint. A few weeks ago I copied core.css from another server to be sure they were identical. They turned out to be exact but it seems using Windows Explorer to copy files to those directories resets or deletes permissions (still not exactly sure what happens here, feel free to comment). Anyway the morale of the story is when copying files use the command line instead of Windows Explorer.