There is not much else to elaborate on the bug, it's exactly as the title states. It is an extremely rare bug that i have seen only a handful of times, but occasionally when using nunu's w, when the button is let go it will simply disappear off the map and not roll forward. I have had this happen at both the smallest size, and the maximum size. As an estimate, it seems to happen only 2-3 times every 100 snowballs you roll.
Unable to provide a video as of right now as i am in a game, but i will as soon as i can