Fluids, I keep running into this issue and it has to be a bug.

[onewheelskyward]

Chem factory pushing out light oil, pipes pushing light oil, tanks carrying heavy oil. Why do I continually get this error?

[DarkShadow44]

Probably have a funky connection somewhere. Post the save, and I'll figure it out.

[DaleStan]

You probably have something (water?) polluting your light oil pipes. Click on one of the tanks. If there's a small amount of whatever pollutant, click the trash can next to that fluid. If it's a significant amount, or if the unwanted fluid comes back, you'll have to hunt through and figure out where the fluid networks are connected. If you post the save, we can help you find the problem.

[Nosferatu]

I see one issue:
There is a chem plant on the bottom left.
It's intakes both demand light oil but one has water connected.

[onewheelskyward]

Ooh nice spot. So water on an _intake_ on a chem plant can pollute the other intake? Honestly, fluids shouldn't flow that way. I'll try it and see if it works. That plant flips between light oil to gas, and light oil to rocket fuel. The weird thing is it doesn't foul anything existing (note only light oil markers on everything connected) just new connections. I still say it's a bug in the fluid detection code.

[SoShootMe]

So water on an _intake_ on a chem plant can pollute the other intake? Honestly, fluids shouldn't flow that way.

It makes sense that a chemical plant set to a recipe with a single fluid ingredient (such as solid fuel from light oil) has both fluid inputs connected together internally, from which it follows they are both part of the same system (one of those that the error refers to), just like pipes joined externally. I think this is basically how the game models the situation, and is why it is not a bug.

Don't get me wrong, there is still apparent "weirdness" because this means that (for example) if you connect water to one of the inputs in this case, the other one can only be connected to water even though no water is in the chemical plant (nor will water flow between the inputs).

The were more strict checks on fluid mixing in the past (I'm not certain but I think you would not be able to change the recipe if a fluid input was connected to the wrong fluid for the new recipe), but they were problematic and pared back. It's best to treat the error unexpectedly occurring as "Something is definitely wrong with your setup" but note this does not mean that if there is no error, there is nothing wrong (nor does it mean that it won't do what you want even if there is something wrong).