The Return of the Archons - S1-E22
Other mistake: On the bridge, as the camera looks at the view screen, there is a flashing blue light reflecting on the console in front of Sulu, but there is no light other than scanner under the viewer flashing and it's too far away to project light that far, and it's amber colored.
Chosen answer: This was an early episode and TV shows often make small changes to set design, props, equipment, and so on as the program progresses. Most likely it was felt that this particular prop was not effective and the producers decided to eliminate it.
raywest ★