When you try to duel the enemie ADC as Sion
But he just finished his 
http://photos.vanityfair.com/2015/06/01/556bb676378caf0d670e4d50_jon-snow-white-walker.gif
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And then you realize that extra damage has a limit and you need even more to kill him.
This is why Sion doesn't duel
That's... Supposed to stop Sion? Hahhh. I've 1v1'd a fairly fed Vayne as Sion before. Granted, she rolled INTO my ult, but.