Ah!! Had me confused with the 'horizontal' ref. Thought they might be what you were referring to but they weren't horizontal. Was beginning to think I was getting low on coffee! These (I believe) are part of the duplicate render interaction flaw. You will find this in most of the images I have posted above. I have had these many times in this sort of situation in other images. I found, via test images I had not posted here and in some of my image models where this was seen, that this is cleared completely by the randomize process.

Pass no temptation lightly by, for one never knows when it may pass again!