While lagged out during a Hexakill game, I noticed that the shadows - specifically character shadows -were flickering. From what I can see, it appears to be an issue across each map type.
1. Start a game
2. Have shadows enabled
3. View flickering shadows
It can be seen in this video from Surrender@20 (The video is displaying the changes to Akali in this most recent update to the PBE, but the flickering shadow is readily visible throughout)