While upgrading a plugin for WordPress (the blogging software we use on this site), I saw the following error: “Briefly unavailable for Scheduled maintenance. Check back in a minute”.
This is a new maintenance mode (introduced in WordPress 3.0) that is activated while upgrading WordPress core and plugins. While this is a feature, there are times when it causes trouble. For example, if your plugin upgrade fails (due to permission error, or in my case, a poor internet connection), the site might be stuck in maintenance mode and remain unaccessible.
To fix this, navigate to your WordPress root (the folder which contains wp-includes). Remove the .maintanence file and the site should be back up.