Wrong/outdated message shown on /user when logging in

I got the following message when I went to /user to log in. Note that I did not POST anything yet, the message appeared as soon as the page loaded.
Also note that my username is not Flexsin (nor did I ever enter it in the username field), so it can't really be a browser caching bug.
It doesn't really bother me, but I thought I'd open an issue in case this is in any way of interest to the webmasters. If not, feel free to close this.