When you are blueprinting assembling machine with a recipe with fluid input (like Electric engine or blue circuits) it shows an input and an output.
So it looks like
It's a very minor bug
edit: as people on reddit pointed out, passing through it's shown as a double arrow, so I assume it just should not show the output arrow (and connections to pipes).
Here's a picture showing the problem:
Edit: I made another test to check barreling and unbarreling, and it looks like there is the same inconsistency