Read on to see an interesting glitch. I tagged the major spoilers, but the description of how to see the glitch gives away some locations involved in the main quest, so caveat emptor. I'm playing on 360 at the moment, so I'm not sure if the same thing happens on PC, but it's worth a look.
To see this you need to be fairly far along in the main quest (spoiler:
after escaping from Raven Rock but before reporting back to the Citadel
). At that point, I wasn't in much of a hurry to continue the main quest. I hadn't finished exploring Vault 87 (for reasons that are obvious if you've played the main quest up to that point), and I still had a map marker for it that said "you haven't discovered that location yet". So, I went back to Vault 87 the same way I did the first time and continued poking around. The door that previously required a key (spoiler:
the one the Enclave guys who kidnapped me came through
) was now open. I explored a few rooms and found the vault entrance. I activated it, assuming I would come out on the world map at the Vault 87 map marker, but I was wrong (spoiler:
you end up in a featureless void
). Apparently Bethesda didn't expect anybody to do that.