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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If I'm with randoms? I just leave. If I'm with my clan then I'll tell them I have something to do. If it's one of those latenight raids where people have been drinking, I just stand still or fall asleep.