Honey blocks cause the sloped face of flowing lava to not render.

When lava flows underneath a honey block, the top face becomes invisible, allowing you to see clearly under the lava through the gap in the texture. In my testing, only honey blocks displayed this behavior, however my testing was only moderately thorough.
I was already able to utilize this to locate ancient debris under the lava, and I imagine that this could be used in other nefarious ways.