Just played a game in which an opposing Gragas was standing on top of me as Poppy, and my W did nothing to him while it was on before he E'd. Which is slightly understandable as he didn't move anywhere, but there was another time he jumped at me, my W inturrupted him, and his E still hit me (both effects took place, both getting Courage of the Colossus). Seems clunky and unintended, especially the second scenario.
Gragas AoE on E extends his visual model quite a bit so this is what causes these problems. Currently on PBE Gragas got reduced E hitbox on its end, maybe it will fix the second scenario.
Yeah Gragas E can be tricky, you can flash behind a ulting 
awesomely dodging, and then theres.... 
with an old 
crystal arrow ludicrous hitbox, you flash behind him awesomely dodging, annndd he body slams the air .5secs after ur sick dodge causing you to get knocked up. You ponder lost in thought of that sick cruel joke of a hitbox.
