A well known bug that I've strangely had opportunity to explain three times in the last couple weeks. 

Summary:

Everything you have with shadows enabled fits in 10 inches, and you're using depth-mapped shadows.

Change the conditions and it will stop. Bigger object, another small object more than 10 inches away, or use raytraced shadows.

SM is not interested in fixing this. 

Note to helpers who keep suggesting a figure:

It's not because it's a figure that fixes it. It's because any figure is bigger than 10 inches. A tiny figure that keeps the whole-scene bounding box within 10 inches will still produce the problem. Please stop suggesting the inclusion of a figure.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)