Temporary user profile created on login
Permissions of existing user may not allow them to log in, or NTUSER.DAT may be missing. This will usually create a new folder in C:\Users named like <username>.<domain>, for example myuser123.DOMAIN. Observed outcome is user missing files in their user profile, which is very confusing for end users. This can stack if it happens again, creating more user folders.
Most easily solved by either resetting permission inheritance on user folder, or deleting via sysdm.cpl after copying any important files out.