Whenever someone friends me and I accept, the user is just called “???” and they’re mii is grey. then I try to unfriend “???” and it says this:
Error Code: 199-9998
An error has occurred.
Please turn the power off and try again.
when I turn it off it’s fixed but jeez it’s annoying.
This is a known issue.
After adding a friend, both parties need to restart their Wii U’s to sync the friend acceptance in the server.
It’s known, but is relatively harmless (only tedious to restart).
Thanks! very helpful!! 
Creepypasta material 
?
I honestly never tried unfriending the user whenever that happened. Glad to know what happens now.