I don't know if it's a bug or if it has to do with zhonya mechanism, server lag or what. But it felt pretty stupid during the game.
This was the match: http://matchhistory.euw.leagueoflegends.com/it/#match-details/EUW1/3019871532/41690966
Similarly you can Lissandra ult after blinking into a Soraka pool. I'm convinced there is a slight delay on the silence when regarding blinks and instant movement.
The zone only checks every .25 seconds or so. If you hit in between one of the silence checks you can get away with stuff like that. I'm not sure it's a bug so much as a limitation of the current system.
