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Today, while playing a game for my first win, I noticed something strange. When I hovered over my passive to check how much health I was regenerating every 3 hits (It was my first time as Xin, so I was unsure of the amounts), I noticed that it was instead showing the tooltip for the buff of Battle Cry. After fighting with it for a few minutes, I determined that it would not change. However, after dying, I looked again, only to see that it was now showing the tooltip for Silence (as I had been hit by Cho'Gath's Feral Scream shortly before dying). I decided to ask a friend in the same game, who was playing Jax, to check his passive. He reported it being normal, leaving me to assume this is a bug with only Xin Zhao. I tried to screenshot it, but as it turns out, Win7 ignores LoL on a print screen, and the resulting image showed only my desktop.
TL;DR: Xin Zhao's passive tooltip is, for some unknown reason, showing the tooltip for the most recent thing added to your buff/debuff bar, regardless of whether it's still there.