The City on the Edge of Forever - S1-E29
Corrected entry: After extinguishing the fire, Spock pulls a large vacuum tube out of the primitive computer power supply he's constructed on the bed. When the shot changes, the tube is back in place again. (00:30:00)
Correction: Before Spock removes the burnt out vacuum tube he picks up a new one which he replaces it with while the camera is on Kirk.
Peter Wallace