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 the group is argumentative. Just leave. If they are nice and you just can't seem to beat it as a group, just say you have run out of time to play. Really any excuse will work. Not like anyone will check up on you. I always just try to be nice. But I have only had to leave a few groups.