Wow, this need some serious updating.
Here's some.
-When exiting out the back of Helios ONE, sometimes you will hear extremely loud metal sounds, like the collision sound, and a bunch of random enemies will appear right in front of you and become hostile. Some enemies/NPCs that appear include Old Lady Gibson and her dogs (No really. And the dogs become hostile to you.) and Cazadors. I am not kidding. This is no joke, it happened to me several times, not only once, and is even listed on the Helios ONE page of Fallout Wiki.
-When your companions are poisoned by Cazadors, healing them with cause them to LOSE HP, and if it goes below 0, it will give them negative health and upon leaving the companion wheel, they will drop dead. You must let the poison die out, as the dumbass companions never use antivenom when in their inventories, and you can't administer them directly. If on Hardcore, avoid Cazadors with your companions at all cost, as more often than not it will result in them dying permanently ("Essential" meaning no permanent death is turned off on Hardcore. When your companions die, they drop dead, forever.)
-Occasionally, Isaac will disappear from the Freeside branch of the Gun Runners, as he presumably walks off to a random area and never returns. If you need him for one of the unmarked quests he's involved in, look around the Hestetler (Don Hestetler or the Crimson Caravan's house. I can't remember exactly where it is) housing area, and according to wikia, he should be there.
-If you are in the middle of Volare! and get Young Hearts, but decide not to complete the previously stated quest and complete Volare! instead, Jack and Loyal will disappear from the hangar permanently and you will be unable to resume the quest. He is supposedly stuck in the "old hangar" with the deconstructed B-25, and can only be retrieved using console commands. I was unfortunate enough to be plagued by this and found out too late. DAAAAAAAAAAAAMN.
I know I've run into more game breaking bugs than this, but this is all I feel like typing now.