This is probably for the DAQ section, but if you are in a quest and reach a point where your choices are complete the quest or goodbye, and you say goodbye, is the quest gone, or can you come back?
You can come back. When it is a significant quest (big xp), I will often do that so I can set up one of my bars with skills I want to level. Then talk to the target again and complete the quest. Then remember to reset your normal skill on that bar! Lol.
You can talk to the NPC again and complete the quest. For example, if your inventory is full so you can't accept the reward item, you can say goodbye, clear the space, and then come back and accept the reward.
In some cases you might not be able to access extra dialogue options if you do that, but I think that's the only thing you might "lose" by doing it.