TFS error 203208: You cannot create a branch at XXX/YYY because a branch already exists at XXX

Question

TFS 2010 is giving me this error when I attempt to move a branch to another project. I totally don't understand the error though. There is some attempt at exaplanation here:

http://social.msdn.microsoft.com/Forums/en-US/tfsversioncontrol/thread/ec04873e-80ab-48cf-be94-96ffb0bf347aTF203208

which makes zero sense because I am not attempting to put a branch inside of another branch! one of the sibling folders at a higher level might hold a branch too but I don't see how that should affect my folder.