We've got a pretty major bugfix planned a few patches in the future (we're hoping 4.18), but wanted to get coverage on it as soon as possible. A TLDR of what we're doing:
Since launch, there's been an issue with overlapping crowd-control abilities where if a champion was affected by multiple CCs they could use abilities after the *first* CC wore off (even if they were still CCed by others). Meaning they could Flash even while stunned or silenced.
A better way to explain through Cho'Gath: when he was comboing his Rupture + Feral Scream on an opponent, they'd get double debuffed with a knockup (Rupture) and a silence (Feral Scream). After the Rupture debuff wore off - even if they were still silenced - Cho's victims could flash away due to this bug.
It's a little complicated but the end result for you guys is that stacking crowd-control abilities on someone will be far more reliable than it currently is. There are a few fringe abilities still doing this (tracking them right now!), but if any of you run into issues with CCs, please feel free to let us know. Thanks!