[GAMEPLAY] Alistar Headbutt bug

I've played League a long time ago, but the upcoming Alistar visual update made me remember to a bug with him. With his Headbutt ability he can push away units, without moving into melee range. This happen to me when I've used it on an enemy minion, which was standing close to enemy champions, used it as smart casted on the Howling Abyss ARAM game mod. I never tried to reproduce it bug it happened in multiple games, but I think it didn't happened multiple times in the same game. This is what I can say about this bug. It might not be accurate or proper enough.

Assumed steps to Reproduce: 1.Start a game as Alistar on the said map and game mod. 2.Level his W ability. 3.Use it on minions, pets and enemy champions for further testing.