I've noticed a rather annoying trend recently where youtube starts a countdown to the next video at the end of our current viewing and starts playing it automatically. Sure, you can click cancel but what happens if you're not there?
What happens if you're watching a video and then you get a phone call and suddenly have to leave your desk.... Don't worry, YouTube will happily continue downloading random streaming videos in the background. Even better, if you've left home for the day (or work for a weekend) you can expect a nasty bandwidth surprise.
That's why you need to turn this horrible new feature off.
How to Turn AutoPlay OFF
In your YouTube account, start playing a video, any video. You should see an AutoPlay option in the top right hand corner. Slide it to the off position.
That's it. It seems to be a setting which is remembered... of course, that's only if you're actually logged into YouTube. If you're not logged in then you'll probably find yourself having to turn things off every time you restart your browser.