Final answer:
In the episode "Heavy Competition", Michael wrote the word "dummy" on the back of Dwight's Rolodex entry.
Step-by-step explanation:
In the episode "Heavy Competition", Dwight finds out that Michael wrote the word "dummy" on the back of his Rolodex entry for the name "Schrute, Dwight". The Rolodex is a rotating file device used to store contact information. Michael's prank on Dwight highlights their comedic rivalry in the show.