In the forum post Getting Latest Version Error (TF30063), a user was getting the error message "TF30063: You are not authorized to access Microsoft-IIS/6.0" when getting files from version control. I asked a bunch of questions, which the user answered, but that didn't really help. The user end up figuring it out. It turns out that the client computer had the Google Web Accelerator installed, and the web page caching was causing problems.
I had seen anti-virus applications and firewalls causing problems (for example, random files being left writable after a get), but this was a new one. In the Preferences section of the help page for Google Web Accelerator, it describes how to specify sites that shouldn't be accelerated. If you use the accelerator, you'd want to add your Team Foundation Server application tier to the list of sites not to be accelerated.
Alternatively, setting the BypassProxyOnLocal registry setting as described in How to improve command line performance would probably fix the problem as well. Turning that on causes the TFS client not to use the IE proxy setting.