Guild Wars 2

You know, when you break out of a map (Intentionally or unintentionally)? how does it work in GW2 or any MMO from a programming or coding standpoint? I know in old 3d one player games it more commonly referred to as parallel universes or secret worlds.

I’m just curious if I want to design a game in the future, how to not have it happen, or if it does why doesn’t the game just crash or snap you back into the playable area, like many games of old?

Most games have what we call a Skybox. A huge cubicle of space in which “matter, space and time” exists. Of course, when game designed create a world, they create a landscape, and then design it in such a way you can’t leave the designed area or see the Skybox. Sometimes you can see the Skybox if the game is really slow with loading, and textures hasn’t appear yet.
When Sometimes, the devs have a slip-up, and there’s a gap in the designed cubicle within the Skybox. Then the player needs only to get through that gap.
In some cases the Skybox is filled with air, you fall down, until you hit the bottom, after which the game resets you to a last solid position. Sometimes the Skybox is filled with (invisible) water, after which you can freely move around.

What you can do is, as GW2 has in some areas “you are leaving the playable area. Return now or you will be killed”. You can create a cubicle of this rule around your playable cubicle so that when players break out into the Skybox, they can only move away a few feet before being reset.