If there's a hole north or south of the block below the one the it's on flickering occurs at the bottom. If a block is placed on the top a glass pane flickering occurs at the bottom. The camera must be moving for flickering to occur. No blocks can be adjacent as it only occurs when in the + state.

To reproduce:
1. Place a glass pane and iron bars, make sure they aren't adjacent to any blocks (including each other).
2. Make a hole to the north or south of the block below the pane & bars.
3. Move the camera (look around).