Ekko Q bug
I've noticed it a couple times - his Q can apply two stacks of his passive, right? Once when it goes out, and once when it comes back. At the end of it's trajectory, it "pops" and comes back. If it misses you on it's way out but the pop hits you at the tip, it will apply two stacks, even though it's only hit you once. Maybe it's not a bug, but if it's not, it's pretty counter-intuitive.