When placing Glass around a Volcano's main lava shaft as an aesthetic touch, random lava textures appeared. The lava does not inflict damage, purely a visual bug:

Minecraft v1.6.4 (An old version, I know), part of the "Test Pack, Please Ignore" Technic Launcher modpack.

Fancy fluids will show this behavior. You can disable it in the config file. If a block misrepresents a side as being non-solid when it should be, the mod simply assumes the fluid can occupy the adjacent space. But, there's other issues as well, like determining fluid heights per corner which I simply choose to ignore. It's just a fun aesthetic thing, but only intended for stationary, non-flowing fluids.