Not sure if this is intended or not, don't know how I haven't done this before until now but:
When there's a lava source and you put water in it, it becomes water and the lava just disappears instead of creating obsidian. Works vice-versa.
I don't know if this is intended since the lava/water just simply disappears and water/lava appears in its place, isn't obsidian supposed to be made out of this?

Video: https://www.youtube.com/watch?v=5Saf5AhuqBk