Stacked Pam's and Dwight's desk to form "Quad-desk" is a prank Jim pulled on Dwight when he used Jim's desk to make "Megadesk," while Jim was on paternity leave. It is from "St. Patrick's Day."
Prank Overview
When Dwight used Pam and Jim's desk to form a "Megadesk," for work and gaming, Jim takes it apart. Later, he stacks Pam's desk on his and Dwight's desk to form "Quad-desk," and Dwight is forced to answer phones through a little crevice between the desk.
Dwight is shocked to see "Quad-desk."