If you press Ctrl+Alt+Left Arrow, the screen will rotate sideways.
Why would you ever do this? Maybe you have a desktop and you’ve turned your monitor into the portrait direction. Or maybe you’ve got a laptop and you’ve placed it on its side.
Regardless, to get back to the normal direction, press Ctrl+Alt+Up Arrow.
This one is in the category of “good prank to play on someone on April 1st.”