Well, when you look at corners of vertically flowing lava, some transparent lines appear. They look like there was no behind them. In my screenshots, they look sky blue because behind them there is the sky. At night, they seem to turn black, proving they're transparent. They also move as the player moves.

How to reproduce:
1. Place a block
2. Place lava on it.
3. Look at the lava going down.
4. Move.

Also, if you zoom in at the fourth screenshot, you'll be able to see the some parts are blue (lake) and some are green (leaves).