As for what you need:
Some Tape (Scotch or Packaging, either works)
A mouse (your victims mouse)
Step 2: Setting up the Prank
K, now while the victim's not looking, take a piece of tape about an inch long, then put the tape on the scroll bar or sensor light (scotch tape probably won't work on the sensor mouses, the light will see through the tape).
Look at the pics if you need any help.
Step 3: Waiting for the victim...
do what it says man...
Step 4: And...
Have fun! And try not to laugh too much when your victim goes insane!