Thanks for your reply. That sounds like an end of the game thing.
It is. Before you do the final quest, the game will give you a huge message box that warns you about the end.
I remember something in Oblivion. There was no warning when the Oblivion Gates started to appear. A player just "had to know" that at one point in the game, after a particular quest in the MQ was completed, that the Gates would just show up and for the rest of the game, the player then had to deal with all the Gates everywhere when doing remaing quests wether MQ quests or side quests.
In FO3 there was no warning for the Enclave ... they just showed up after a particular quest in the MQ was finished and the player had to deal with them for the rest of the game when doing MQ quests or side quests.
I can't think of anything like that offhand. I mean, when you do the first faction quest (ie For the Republic/House Always Wins II/Render Unto Caesar/Wild Card) ranger vets and elite legionnaires will start spawning in NCR/Legion bases, but beyond that, no.
If you go down a particular faction route, though, the others will close themselves off to you. In my experience, this will be accompanies by a quest, warning you that if you continue down your path, you won't be able to support Caesar/NCR/House.
So I'm assuming that there is a point in NV where after a certain quest in the MQ is complete, something permanently changes in the game world of NV. That means for the rest of the game afterwards you have to deal with "it" (whatever that is) while doing the remaining MQ quests and all the side quests yet to be done.
Well, upgrading the securitrons could count as something like that, but all it does is make securitrons tougher and change their face. Not nearly as obtrusive as oblivion gates or enclave camps showing up.