Note that the previous render did not have smoothing on. This isn't just about polygon smoothing. It's about the interpolation of surface normals, which happens even if you turn off smoothing for the whole object.

If I turn on smoothing, then I get bowed things as well.

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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)