Thank you, I did not know that.

https://tinyurl.com/factorio-err-heavyoil
the tanks that cannot connect are on the right, close to the save point, like the picture, heavy oil tanks to the left connect normally.
I cannot see any fluid mixing at all. I have never seen it in Factorio.
I have added a waypoint named Error!!! that is ...

I only see heavy oil listed in any of those tanks, and only heavy oil piping is connected to it.

How is that even possible?

I cannot connect heavy oil tanks that are both half full with heavy oil either with underground pipes, normal pipes or pumps, the error I get is - Cannot connect system with different fluids.
Light oil, crude oil and gas work fine, as well as heavy oil connections of a same type on other places on ...

Thank you

Train stops instantly when a player's train is reversing in manual drive mode and changes to a automatic drive to a station that is forward of the train.
That isn't a bug per se, but the train should have realistic stopping action, not a programed instastop when you change from manual to automatic ...