Factorio Forums

You shouldn't be able to connect 2 pipes with different fluids, however if you place a ghost pipe the robots will build the pipe anyway (although the player can't)
To recreate this you just need to put a ghost pipe between 2 pipes with different fluids and let a robot build it
I tried this with different fluids and it always happens (I made a gif of when I discovered this and a small save where this can be tested with various fluids)

It would be considered an issue with the old "hard fluid mixing prevention" logic, but it was causing too many implementation corner cases it was discarded long time ago (0.18.32) in favor of a soft fluid mixing prevention which only checks for some simple cases of player building manually. If you end in a situation where fluids are mixing just remove some of the pipes and maybe even flush some of the fluids by opening pipe gui and choosing which fluids to delete.

As of right now this is Not a bug.