Kalista Bug.

While Kalista is utilizing her passive, post attack movement, she is immune to Blitzcrank's E. Took note of this happening several times in one game, E went on cooldown: no knock-up. I do not have any video or screenshots, but this should be easy enough to re-create.

There was no mention of her passive granting immunity to knock-ups in either the tool tips for the skill or the champion spotlight. So I believe this is not working as intended.