One-sided face/mesh problems, tricks, workarounds, stumbling blocks, etc..

This thread is for ANY discussions on the subject.

It's also a continuation of the discussions started on infinity10's  "any thread discussing waxy leaf surface shader ?" thread that are more specific to one-sided face/mesh than to shaders for realistic leaves. Here's the point in that discussion (part way down page 5) when this new thread was created, and here's the point (near the bottom of page 2) where I reopened that thread and started on about one-sided transmapped leaves.

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The 3Dcheapskate (also available in DAZ and HiveWire3D flavours) occasionally posts sensible stuff. Usually by accident.
And it usually uses Poser 11, with units set to inches. Except when it's using Poser 6 or PP2014, or when its units are set to PNU.