If Singed tries to Fling Cho'Gath it does nothing and Fling goes to cooldown state. This happens I beleive after Cho'Gath's first growth. It is consistent.
Have you seen Cho'gath after a growth or two? He's the size of a house, no wonder Singed can't toss him.
I can only presume that the ability going into CD state is because Singed threw his back out attempting to lift something so heavy.
I actually just commented to a friend yesterday how funny it would be to see Singed throw a fully grown, Feast level 3 with 6 growths Cho'gath, over his head.
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