"My favourite fact about Star Trek TOS is that, because automatic doors weren’t invented yet, the ‘automatic doors’ in TOS were really just some guy yanking a rope and pulley system to make them slide open. Problem was the person was far enough away that they couldn’t see when exactly the doors needed to be open, and relied on a signal from somebody just off set. The actors, however, had to act as if they were 100% confident the doors were going to open at the exact time and moment despite the fact that they occasionally did not which lead to multiple occasions in which the actors walked directly into the doors while they were opening."  via