This is now just a curiosity, as I found a way around the problem.

I was playing with nodes (well, my gf is away for a few days) and after many renders that were fine and dandy, suddenly a vertical line appeared in the reflections of both of these cylinders (arrowed). Not the cylinders themselves, just the reflections. I reloaded the floor texture, deleted and reloaded one of the cylinders, saved the scene and restarted Poser... etc. etc. No change.

Then I tried rotating one of the cylinders, and it disappeared, as shown in the inset render.

Does anyone have a clue what causes this? This is the standard Poser cylinder, scaled up by 300%. I use them a lot, but have never seen this before.

(edit) rotating by only one degree gets rid of the line, but it returns at zero rotation.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)