Under Safari and 10.6, I've noticed an annoying problem: All too frequently, when I go to full-screen on a YouTube or other Flash video, the menu bar 'forgets' to disappear. It's really easy to fix.
Just click on the menu bar! Yup. That easy. Click on a menu, an app up there, or even (as I've found) just on the whitespace.
[crarko adds: I haven't tested this one. I don't really watch that much full-screen video on my Mac, so I don't notice this. If someone observes this in 10.5 or earlier, let us know in the comments, please.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110318070002294