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Summary: You die on nothing, while other curves pass through each other. See also http://forum.curvefever.com/content/dead-nothing. Mostly occurs when switching tabs before/during a game.
Every round I've played today, in the first game the screen I see is misaligned/uncalibrated with the server's view/collision detection. It makes me collide in places where I don't see any other snakes and also other snakes sometimes seem as if they go through each other. After the first game, everything was normal.