It appears it is an old bug, but it happened to me for the first time in my last game 10 minutes ago.
When I dig up Nunu as Rek'sai while he's rollin his ball, his champion model launches into the orbit while me and my teammates can still target him (even tho his hitbox is invisible and his object, a champion model, is at ISS tour) He eventually lands smoothly but damage has already been done. The Nunu player got triggered, rip, but wasn't fun to me either, really unfair!
How to launch Nunu&Willump 101:
1. Play as Rek'sai against Nunu
2. Let him start rolling his snowball
3. Knock him up with your W
4. Press F for respect