I'm noticing that the mycelium block must be directly under the barrel for the water in the barrel to convert into witchwater. This would make piping out the bottom of the barrel rather difficult. Having them below the Y level of the barrel, but adjacent, does not work. I believe this is unintended? It's certainly not the behaviour I am used to, anyway.

EDIT: Oh yes, and it seems that sometimes, it acts strangely. The water seems to change colours to that witchwater colour, without actually changing into it or showing that it is in The One Probe. After that, it begins the proper transformation, repeating the colour change sequence and showing the progress in TOP. Might be a glitch there.