Because I am a shit ;)
I did this all the time to my coworkers when I used to work in a office. Watch them freak out! The only problem is this doesn't work with older computers.
At the same time press all three of these buttons: CTRL + ALT + The "down" arrow
To fix it press: CTRL + ALT + The "up" arrow.
If you do this trick please let me know in the comments how it went. I want to live vicariously through you. :)
PS: This doesn't work on all computers. If you have a little more time on your hands you can also flip the screen by doing this:
This method needs a graphics control panel.
1. Right-click a blank space on your desktop to bring up a context menu.
2. You should see something like "Graphics Control Panel". Select this item.
3. Browse through the configuration pages till you see something regarding rotation. Select the Flipped Landscape option.
4. To undo this, repeat the same steps, except for the rotation option. Select the Landscape option instead.