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Continuity mistake: On Utapau, when Obi-Wan faces off with Grievous, Grievous sends 4 MagnaGuards against Obi-Wan. Obi-Wan Force-pulls something from the ceiling to crush the droids. But in the ensuing fight between Obi-Wan and Grievous, the ceiling thing and crushed droids are gone.
Answer: Darth Vader has a special chamber in which parts of the suit can be removed for comfort. When he is not in the chamber he must wear it at all times in order to survive.