⌘T in nearly any app which uses tabs opens a new tab. In apps without tabs, it opens the Show Fonts. This is a floating window, which cannot be closed with ⌘W, the usual binding to close a tab or window.
While it can be closed in most cases, you have to use a different keystroke, which can be problematic.
Example:
This happens to me several times a day. I’ve noticed that since Leopard enabled the ability to scroll background windows, I do it more. While I love that ability, I think it removes a subtle cue as to which app has the focus. The Firefox 3 betas exacerbated the problem, since they appeared active even when in the background.
Discussion