For a computer programming class, I sat directly across from someone, and our computers were facing away from each other. A few minutes into the class, she got up to leave the room. I reached between our computers and switched the inputs for the keyboards. She came back and started typing and immediately got a distressed look on her face. She called the teacher over and explained that no matter what she typed, nothing would happen.

The teacher tried everything. By this time I was hiding behind my monitor and quaking red-faced. I started to type, "Leave me alone!"

They both jumped back, silenced. "Whaa??" the teacher blubbered.

Then I typed, "I said leave me alone!" The kid got really upset.
"I didn't do anything to it, I swear!" It was all I could do to keep from laughing out loud. The conversation between them and HAL 2000 went on for an amazing five minutes.
Me: "Don't touch me!"
Her: "I'm sorry, I didn't mean to hit your keys that hard."
Me: "Who do you think you are anyway?!" Etc.
Finally, I couldn't contain myself any longer and fell out of my chair laughing. After they had realized what I had done, they both turned beet red. Funny, I never got more than a C- in that class.