Whenever a quest gives an item it doesnt check for inventory space, it simply completes the quest without giving you the items if you do not have enough space. If it is part x/x of a questline then it is fine because you can just forfeit and reaccept, but if it isnt then you're straight out of luck.
For example with the demon hat questline if you complete the job specific part but dont have enough space for the etc, it will simply not give you the etc while still clearing the quest. Now you cant proceed with the astaroth part because you dont have the etc and you cant reclaim it (I'm assuming the quest is otherwise coded into the game properly in the first place)
For example with the demon hat questline if you complete the job specific part but dont have enough space for the etc, it will simply not give you the etc while still clearing the quest. Now you cant proceed with the astaroth part because you dont have the etc and you cant reclaim it (I'm assuming the quest is otherwise coded into the game properly in the first place)