I accidentally click on embedded YouTube videos which opens a new tab of the YouTube page while I was only trying to pause the video. I am not sure if it's possible but it would be great if the video would just pause/play when you clicked on it and maybe also had a link to the YouTube page on it.

From my experience, Greasemonkey isn't very good at working with embedded objects such as flash. My suggestion here is to just send this request to YouTube.

I tried writing a script before to pause YouTube videos in unfocused tabs. No luck. If I recall correctly, code from http://www.squarefree.com/bookmarklets/flash.html didn't work with YouTube, though it did work with some flash content.

That would be so cool considering long videos freeze sometimes and you have to refresh and start all over, great idea