Star Trek: The Next Generation - Two Picards Walking, One Picard Reflected
In the last ten minutes of the episode, the Captains Picard (Prime and Future Picard) are traveling through various corridors and turbolifts. After exiting the turbolift and walking towards the camera down a corridor with red lights in the wall, the two captains then turn and head down a second corridor.
This second corridor has a reflective black computer console lining the wall on camera left. As a result of the split-screen technique used to create the illusion of two Picards, only one of the two men is reflected in the black console. Both the connecting corridor and one of the two captains is clearly reflected, however the second man is not.
Time Squared (Season 2, Episode 2x13)