The lavafabricator stores more than than it could.

I can tell you that I used Hardened Fluidducts of Thermal Dyamics that were connected to Smeltery Tanks of Tinkers Construct.

I am unable to recreate this one and looking through the code the only way I see this could happen is if some pipes tried to take negative amounts of lava from the fabricator.
Would you be able to provide more info on how you got this?