This picture shows the inside of one of the main doors in a large commercial jetliner. After closing the door, the flight attendant attaches the emergency slide on the inside of the door. After attaching the slide, the attendant attaches this red strip across the window. It is meant to signal to a person outside to not open the door. Opening the door could be deadly since the emergency slide would automatically inflate. It seems like it would be very easy to forget to attach the red strip.
Design suggestion
When there is a strong likelihood of forgetting a critical step in a hazardous procedure, it would be better to have an automatic way of doing that step. For example, a better solution would be to have a sign or signal that automatically occurs when the slide is activated.