I dont know if this is a bug or a feature, but in the Portal to the Surface quest, if you talk to the lady outside the building when you finish the quest before you talk to the guy inside, then you can no longer go inside anymore, which eliminates the whole "Let's go in there and kick his ass!" feeling that's usually going around at that point, and also just detracts from the feeling of satisfaction because you feel like you didnt tie up loose ends. It basically finishes the quest whether you wanted to or not, and you just have to wander off.

There's a deliberate dialogue option with the woman that prompts you to enter the area and finish the business before concluding the quest with her. Allowing people who have finished the quest to re-enter the area would cause headachey problems.