Fallout 4 has a secret room accessible to players by using a console command. The room is filled with footlockers, and those footlockers contain every single item in the game. Skyrim has a room like this, too. And, apparently, Fallout 76 has a similar 'developer room' as well, containing all of the game's weapons, armor, items, plans, and recipes—even some items that haven't been officially released yet. There is also, oddly enough, a human NPC standing in there. Weird.
Via this report from Eurogamer, you can see a video of a player exploring the purported 'dev room' here, though I expect it may be taken down at some point so I've made some gifs. You'll see the various chambers, shelves, and footlockers crammed with weapons, plans, food, and other items. The level 1 human NPC, named Wooby, is hanging out in there too.
How the dev room is being accessed isn't known (to me, at least), but it presumably involves the use of mods (in the video, when the player brings up their map, you can tell at least a couple of mods are in use from the map icons displayed) or the editing of game files. There's no cheat console to enter teleportation commands in Fallout 76, naturally, which is the method used to reach the dev rooms in Fallout 4 and Skyrim.
According to Eurogamer, some players have known about this dev room for several weeks. It also sounds like players have not only managed to get into the dev room but also get back out with some of the unreleased items, such as several power armor paint job plans that may be a part of a future Fallout 76 update. Some have apparently even been trying to trade these unreleased items with other players, though mods in the Market76 subreddit have been banning users attempting to do so.
