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A while back, a feature was added that allowed buffering of abilities while CC'd so that the ability would be expelled immediately upon being available after a stun, knockup, etc.
This helped non-smart cast users still get the benefit of getting their desired spell off at the earliest possible moment. However, it has appeared to me through several cases that this buffer can override future cc's done to the champion, most notably, silence.
-Chogath casts rupture on an Ezreal and he is knocked up, and the knockup animation starts
-Ezreal casts his E to arcane shift away while knocked up so that it will happen immediately once the knockup has ended
-Chogath casts W and silences Ezreal during the knockup animation and the silenced text is shown
-The knockup animation ends
-Even though, Ezreal has clearly been silenced, his arcane shift spell still goes off because he buffered it BEFORE he was silenced during the knockup
Hard to reproduce with one person, but I'm pretty sure this is what's going on.
It's annoying to perfectly chain your CC silences with Supports such as Lulu, and Soraka only to see the enemy still get away due to this bug. Please take a look at it.
It's not that the buffered input is overriding the cc, it's that there's a window after any cc where commands are available before the newest cc is applied.
CC the champion
CC is over, enable commands again
Check for CC's that might have happened.
Oh you were stunned previously, disable commands again. CC the champion.
It's that the enable is happening after the check in the code most likely. In theory then, the buffered input has an opening of one frame to do the command before the next cc takes place.
This is likely deep within the gameplay as well as the networking code, so I understand if it would take awhile to fix and maintain this issue.
Edit: Discovered that you can even auto-attack/ use other moves WHILE stunned if you are stunned during one of your own cast animations and buffer the input during your animation http://www.youtube.com/watch?v=drZWTzk9L_E