While trying to access Twitter, this is what millions saw today:

That, my friends, is htaccess authentication, and is often used to allow access to only a select few people to a directory or file, or in this case, a whole web site with millions of users.

What happened?
“Staging” is a development term, which refers to a final testing environment designed to be identical to the “Production” or live environment. After all changes to a site are made and tested, they are often re-tested in a staging environment and finally released into the wild. At a glance, it would appear that someone accidentally copied over the .htaccess file from staging (which protects the staging environment from prying eyes) to the production environment.

I emailed Twitter a few moments ago and will post any response here. Whatever happened, it seems to have wiped out Twitter for a few minutes, but it’s all back now.

On a side note, you can still see the message by trying to access the .htaccess file directly at http://twitter.com/.htaccess Don’t bother, though, because it’s just what’s shown above.

Update: From status.twitter.com

Users are running into a prompt for login on twitter.com. This is because of a misconfiguration error and we’re working to fix it right now