If you right-click a part in Vault Explorer and drag it to another folder, one of the options (when you let it go) is to "share". If you decide later that you don't want the part, how do you un-share the part and delete the copy?
The problem is: When you check out the assembly that the part is used in, the check-out process tries to checks-out the original part and the shared part - but it's the same part because it's shared?! So every time you check out the assembly, you get a warning that the shared part is already checked out.
Peter