You know how Star Trek always has those fast opening and closing doors?

I uses to know a guy who worked on props, and actually helped design and build those doors. He said they were terrifying, as there was simply no way to build them sufficiently light weight to not have the potential to seriously injure someone if they accidentally closed on you. It was bad enough that they had to carefully choreograph ever scene where someone walked through the doors, along with multiple independent people actually operating them to make sure one person screwing up wouldn't be enough to result in an injury.

Again, physics screws you over. A robust door is heavy, and that much mass moving that fast is enough momentum to seriously hurt you if the sensors ever fail.