Fans of Star Trek know that the original series was produced on a shoestring budget. That’s what gave us futuristic devices like the transporter and the tricorder, as a box with a few buttons could do anything in the future. The production crew even had to account for William Shatner’s hairpieces. It won’t surprise you to learn that various devices got re-purposed over and over as different devices over the course of the series. Take, for example, the one called the “CD rack.”
In TOS: "I, Mudd", "The Immunity Syndrome", "Obsession" and (modified) in "Elaan of Troyius" there is a strange strobing device that we call the "CD rack" because it looks like one (yes, we know there weren't CDs in the 1960's). The fourth appearance looks to have been modified with the orb thing from the Romulan cloaking device (later used in conjunction with Nomad to form the robot M-4 in "Requiem for Methuselah").
You might not realize such frugality extended to the later Star Trek series, too. Re-uses spotted by fans from the various Star Trek TV series are documented in a gallery at Ex Astris Scientia.
-via Metafilter