TL;DR
It's really annoying when pipes with two different fluids auto-connect on placing a pipe segment, so have placement logic disallow placing a pipe segment if it would connect segments that contain two different types of fluid.What ?
This is NOT Flow Control (in which one places specially-shaped pipe segment types that don't auto-connect).Rather, Factorio disallows you from placing a pipe that would make a connection between pipes that currently carry different fluid types.
"Single-ended" items like offshore pump, refinery, chemical plant, could still be placed and wreck your network .. I don't see how to stop it, since you can change chemical plant recipe after placement.
If you really want to make the connection, you'll first need to empty one of the networks.
If you like accidents, you can make the connection in the split-second during which the neighbouring pipe is empty but a different fluid type is coming down the line, but it becomes a much rarer event.
Why ?
This mostly-removes the most common "darnit I did not mean to click there" accident that takes ages to recover from.There may be some argument that it's a Dwarf Fortress kind of fun to clean up, but perhaps most disagree and just find it annoying.