In the August 26, 2002, Windows Tips & Tricks UPDATE (http://www.winnetmag.com, InstantDoc ID 26404), John Savill writes about reserved words (i.e., words you can't use) in DNS zones. The file system reserves the same words. To verify this fact, try creating a new folder and using one of these words as the folder's name.
I recently discovered that user logon IDs also shouldn't use these words. I created a new user named Connie Nielson, with the user ID con. The creation went smoothly, but the user subsequently had problems with her profile and home folders. The profile folder had been renamed to \\server\profiles\con.pds, and the home folder was missing. I had to rename the user ID, but I was never able to delete the con.pds folder.