When you create a user, Windows also creates a home folder for that user. Traditionally that home folder is named same as the user. So the process of creating the CON user expected to fail when creating their home folder because that name is not allowed in the file system.
Also note, that Windows' filesystem is case sensitiveinsensitive, so "con" and "Con" have the same issue.
I am not sure, of if you can create another user name and then rename the user, because there are potentially other places, which that will conflict with the name. The general advice is to stay away from the reserved words - the benefit is most likely not worth the fight.