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Here is the official word from Microsoft TechNet, The C:\Users folder is shared when any sub folders from C:\Users\<username>\ is shared. The article is stale and mostly unhelpful, but worth knowing to complete the story.
A sharing violation occurs when someone other than the owner tries to access a file. A sharing violation exists to protect those who use a networked system or a computer with multiple accounts. Shared violation indicates someone other than the owner of a file or folder is attempting access. An administrator, or a member of the administrator group, can acquire ownership of any file in the system. This allows access to files in case a password is unavailable.