In https://en.wikipedia.org/w/index.php?title=User_talk:PrimeHunter2&diff=next&oldid=760171056 my PrimeHunter2 account saved the line:
:Pinging [[User:NoSuchUser]], [[User:PrimeHunter]], [[User:PrimeHunter3]]. ~~~~
The account has both "Failed mention" and "Successful mention" enabled at https://en.wikipedia.org/wiki/Special:Preferences#mw-prefsection-echo
It produced a notification to the author saying:
3 notifications about mentions you made on the PrimeHunter2 talk page: 2 not sent, 1 sent.
Username does not exist: NoSuchUser
You mentioned: PrimeHunter3
You mentioned: PrimeHunter
"2 not sent, 1 sent" should have said "1 not sent, 2 sent". The rest is correct. I own the accounts. PrimeHunter and PrimeHunter3 both received a notification.
The bug was discussed in January at https://en.wikipedia.org/wiki/Wikipedia:Village_pump_(technical)/Archive_152#Notifications