In current version (as of March 30th); if an administrator tries to edit a user name that includes an apostraphe (') then the updateuser() function of modules/system/admin/users/users.php will treat the user name as:

"my/'user"

instead of:
"my'user"

In my case, this caused the function to throw an error. I found it could be solved by adding the following (current cvs line number):

Line 179: $uname = str_replace("\\", "", $uname);