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 need to leave it's usually due to a really shitty group, or a group full of squeekers. Either way I just shut my ps4 off, and go on with life as if I never was in the group in the first place.