Factorio Forums

The upgrade planner allows converting between regular pipe and underground pipe. This allows you to place an underground pipe where you wouldn't normally be allowed, connecting two distinct fluid networks. It shows fluid from one and tinting from the other. Building more pipes causes it to flip back and forth.

Restricting the upgrade planner UI to not allow alternating regular pipes/underground pipes would solve it for vanilla although mods with multiple underground lengths would still be an issue.

In fact, I am able to replace an underground pipe with a regular pipe by hand even if this would result in fluid mixing. Luckily, we thought of saving before trying it out

Good find. It's funny we missed this one. Problem is that the pipe underneath is connected to the neighbours, therefore the new, simulated pipe does not connect and the fluid mixing check does not see them. And besides, the upgrade planner was not checking for fluid mixing at all. Fixed now.