In the early part of the game, there are safety posters all over the Mother Base. If you stand next to one of the poster, there will be an audible sound effect and a green box will drop on Big Boss' head, knocking him to the ground. There are several of these posters that scattered the Mother Base, but it will only work one time per session.

As showcased in a YouTube video by user Blynn Beltran. Metal Gear Solid 5: The Phantom Pain Falling Box Easter Egg

https://youtu.be/tSvh9jolsTg