Lag, Fail, Teammate, you name it. Video evidence is always best.
(It's hard to explain lag deaths without a video. "Uhh well I popped my super then he kinda body-slammed me.")
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I don't have a clip of it, but I was playing on rusted lands and I was running to the heavy ammo and my character just fell over and died. No one killed me, nothing touched me. I just creased like paper and died. It said misadventure. I think my character had a stroke or something.