Some server configurations, typically running Pure-FTPd, have trouble updating WordPress via FTP. When doing an upgrade or a plugin install things may start fine but fail in the middle. It’s a bummer during an update since your site is also in maintenance mode and visitors can’t reach your site. Maintenance mode will timeout but to quickly get out of it just delete the .maintenance file from the root of your website.
Common error messages might look like this:
Could not copy file.: /wp-admin/css/theme-editor.dev.css Could not copy file.: /public_html/wp-admin/css/theme-editor.dev.css Could not copy file.: /.../.../wp-admin/css/theme-editor.dev.css Could not create file.: /public_html/
And then you’ll probably see an Installation Failed message.
There is a patch going into WordPress 3.2 that should permanently take care of the issue and the plugin will be obsolete after upgrading to that version. In the meantime, installing and activating the plugin should make upgrades work as expected.
Download the FTP Upgrade Fix plugin from WordPress.org or search for it within the WordPress admin. You may have to install it manually via FTP if you’re also having trouble installing plugins through WordPress.