I took a college course in computers where we learned how to put together PC's amongst other things. Each of use had a Pc of our own to use and each was labeled clearly with a number. A guy, lets call him Pete sat in the row behind me and was a bit of a dick.

One day Pete decides it would funny be the swap the tower of his computer with mine while im at lunch so when I come back I turn on my pc and am confused at to why I can't login with my password. Pete stands up with a Got'cha! and laughs.

I wanted to get revenge but knew if I swapped the towers like he did he would be expecting it and he would look at the number to check first. So i decided to use the my new found skill i had just learned in class and decided to swap the hard drives in our computers so when he turns on his pc his tower it would boot up and he wpuld have the same trouble i did. Not just that but after swapping the hard drives i then swapped the towers again so that when he sat down he looks at his tower he sees that the number is wrong, he laughs thinking he has caught on to the prank and immediatly goes and swaps the towers back, so happy with himself for not getting pranked. Reconnects the tower boots up and cant login! He went from smug to furious!