It's just one example of what can go wrong, and it requires specific conditions (building up certain portions of all three with significant populations.)
The rest is just for the curious.
Want another fun one? If your settlement consists of multiple cells (invisible loading 'grids'), and you have things built in a certain order, your settlements can get... weird. Sanctuary is an easy example again, due to its size.
Cells: All ES/FO (plus Starfield) games from Morrowind on are built in a grid. Think of it as graph paper. You're standing in one square, and the game loads X number of squares around you. Cross into the next square, it unloads a row behind you and loads a new row in front of you. It's like always walking in the center of a giant spotlight that moves when you do. Those who remember playing Morrowind on the hardware at the time may remember that when you were traveling, it would pause for a couple of seconds every few minutes. That was you crossing a grid line and waiting while it loaded/unloaded the area. Loaded areas have everything - all the creatures, all the scripted events, everything that needs kept track of, etc, active. Unloaded areas are just empty terrain. Example of the full Fallout 4 map, showing the grids, spoilered for size:
Here's how it loads the squares, if you have the number set to nine.
You're standing here. The square you're in, plus the squares around you (nine total) are fully loaded - and outlined in red. Everything in them is active and loaded. Everything outside of them - like the edge of the city to the north - is inactive.
You walk north. When you cross the grid line, the squares in front of you, with all of the buildings, get loaded (green +) and everything related to them becomes active. The squares behind you, with the other buildings to the south, get unloaded and all of their processes go into standby mode (red hatch marks.)
That's the whole system.
Here's the Sanctuary triangle of death. The three houses are Sanctuary, Red Rocket, and Abernathy Farm. In certain locations, they all become active, as shown.
If they're all fleshed out with high population (let's say your character has a Charisma of 8, giving you a max of 18 settlers), then standing there means that instead of 18 settlers, the game is keeping track of 54 of them, plus all of the scripts, plus all of the turrets, and farms, and production facilities, and bloatflies, and molerats, and...
This gets worse if you increase the build limit or have mods with additional scripts. As a general rule, only build two of those three settlements, and two of the Castle/Spectacle Island/Warwick set.
Anyway, I mentioned something about weird things that happen in multi-cell settlements. Here's Sanctuary.
You have most of your defenses - turrets and spotlights - by the bridge (T = turrets), your farms up by the cul-de-sac (F = farms), and the G is your generators.
You approach from the south to the point in the picture. Guess what - all of your turrets and spotlights just failed. Why?
Because your generators haven't loaded in. The turrets don't have power. Cross the grid to the north and the generators will load in, but your defenses may or may not start functioning. A lot of the time you'll have spotlights, for instance, just stop working forever until you scrap them and rebuild them. Also, if you pull up your Pip-Boy and check the settlement, it may show that your settlers are starving. Why? Because you haven't loaded any farms. What's worse is that if you turn around and leave, the game thinks that's the actual state of Sanctuary - settlers, but no food - and you'll start to lose population until you go all the way into town and let it load everything properly.
Now, looking at that same picture, pass by Sanctuary to the northeast without going all the way in. You just loaded your generators and farms. Yay. But you're just passing by. Then, out of the blue, you start getting notifications about Sanctuary being under attack, and by the time you make it back, half of your stuff is damaged. But you have Sanctuary so well defended that it never gets attacked! Why did you get stomped?
Because you loaded in all of your resources, but none of your defenses. If your resources are higher than your defenses, you get attacked.
And note that everything I showed here is present in the base game. Mods aren't a factor (some may make it worse, but others may make it better.)
When I said in the Starfield thread that the engine was deeply flawed and really needs to be replaced, I wasn't kidding.
