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.  "What the... " the teacher said.

I typed, "I said leave me alone!"

The kid got real 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 the computer went on for an amazing five minutes.

Me: "Don't touch me!  That hurts!"

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.