Anivia's R had a bug with her R that was/will be fixed but it wasn't a bug (more info in post)

Bugfix states: >Fixed a bug where several forms of loss-of-control effects (Fear, Flee, Taunt, Knockback, and Charm) weren’t properly cancelling Anivia’s R - Glacial Storm

It is bugfix when it comes to Knockback, but all other of those CC have in their description that they WON'T INTERRUPT Anivia's R - Glacial Storm.

Here is evidence: http://leagueoflegends.wikia.com/wiki/Types_of_Crowd_Control