Have you ever had those raids where your fire team just doesn't work to you. Either if they just suck, or one of them dropped the n word. You just start thinking HOW can I leave politely or what can I say that is really random and stupid and will confuse them to make my escape? I've gone between saying I've played too much destiny and my eyes hurt or my hamster just got the news that he's been accepted to university, depending on the group.
EDIT: if you want Leave your best leaving lines, serious or not.
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when I want to leave I just leave. i don't say anything or do anything I just go to orbit and I am gone and done. if in a party I leave it before I leave the raid and then just ignore any reinvites to party and game at that point.