Username Funny Stuff

I just got a new Mac.
My old File Vaulted Mac booted into the Lock Screen which showed my FirstName LastName filled in as the (only) user and under that, a box to enter my password.
After successfully setting up my new Mac, I tuned off. Later on power-up, it booted into the Lock Screen which showed two empty boxes-— the first requesting my username and the second my password.
I entered FirstName LastName for the username and then the correct password. No go.  I got the shaking password error and couldn’t login. Retried several times, checking for caps lock key, tried a different keyboard, etc. No joy.
So I went into the Change Password sequence, logging in easily with my Apple ID  and successfully changed the Mac password. On boot, still no joy- couldn’t login.
Eventually I called tech support and an Apple Babe walked me through changing the password again even though I told her I had already tried that. She insisted I do it again and while doing so, I noticed that my actual username was in fact firstnamelastname— no caps, no space.
I told her that and we abandoned the change password operation and rebooted  entering firstnamelastname as the username. All worked fine. Login!
The root cause of the problem was when I set up Lock Screen in System Settings-- selecting for "Login Window Shows":  "Name and Password" instead of "List of Users"-- the latter which pre-enters firstnamelastname even though it shows FirstName LastName.
Live and learn....
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There’s no problem so bad that you can’t make it worse.
-  Astronaut Chris Hadfield