I had never seen this one

A while ago I had to log in. After entering my user name and password and clicking to proceed, I received the following error message:

However, my avatar and correct name in the Welcome message seem to indicate that this user had been found, and even logged in! I don't remember on what I clicked after reading this message, but I found that I had indeed been logged in, and everything was working normally.

Has anyone else experienced this?