Add &flip=1 at the end of a Youtube video URL to make it upside down, this effects the video too, or &flip=0 to put the page and video back to normal
If you've gone to Youtube for the first time today, the video pages will already be upside-down, but to the right of the video, you'll see a button that says "I want to use the original layout"
I believe next to last year's April Fools, where all the front page videos were Rick Rolls, this is probably my most favorite Youtube April Fools joke :D
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