Factorio Forums

Demonstration video: https://youtu.be/30K6i7LMQRI

What did you do?
1. Place underground pipes in a spot where replacing them with a pipe would connect it to Fluid A.
2. Drag pipe filled with Fluid B onto the underground pipe.

What happened?
Depending on the direction (relative to the underground pipe's orientation) you drag the pipe from, the underground pipe will either be replaced or will not be replaced. More specifically, treating the above-ground side of an underground pipe as North, the pipe will be replaced if you drag from the North or West, but will not be replaced if you drag from the South or East.

What did you expect to happen instead?
Underground pipes should never be replaced with normal pipes if it would allow two fluids to mix, no matter their orientation.

I am not considering this to be a bug after old "prevent fluid mixing at all costs" approach was abandoned for the "simple fluid mixing prevention" which handles some of the basics cases. If you have mixing fluid then cut some pipes, flush wrong fluids if they entered wrong system and life goes on. IIRC this is a corner case of the simple fluid mixing as it does not remove entity which will be fast replaced and because of that some fluidbox connections are occupied preventing the check of one of those fluid systems. Maybe it is a minor issue, maybe not a bug.