If you’re working with TFS and automatic builds of Silverlight apps, you may happen to stumble upon this error in the end of the build:
Copying file C:\Builds\2\xxx\yyy\Binaries\zzz.xap failed. Access to the path 'C:\Builds\2\xxx\yyy\Sources\www\ClientBin\zzz.xap' is denied.
The reason for this error I’m not 100% sure of, but it seems that the target file is read only and the build service do not have the proper access to overwrite it. Others have had the same problem with this and the suggested workaround/solution is to look up the \ClientBin\zzz.xap file in Source Control Explorer, right click to bring up the properties and the Security tab, then mark “Deny” to read for the Build Service. I did it for both the Builders group and the project collection build service accounts:
I found the tip for the solution here.