If Kayn ults Fiddlesticks while Fiddlesticks is using his W he can hurt Kayn inside him.
If Fiddlesticks is sucking Kayn, and then Kayn goes inside of Fiddlesticks, then Fiddlesticks will continue to suck Kayn, even though Kayn is inside of him. It sounds dirty, but I swear, it's a bug.