OK I've found something else completely strange.

PJ - you said based on which way a normal faces, it may or may not let IBL light through under IDL.

How does this grab you?

Load a subject prop. Set up a constant light gray IBL, no shadows no AO. Render. The prop looks uniformly lit.

Load a one-sided square. (EDIT: MAKE IT BLACK) Scale it up 500% (EDIT: I said 5000% first time - I meant 500%) and position it beside the prop so you can see the prop, but this wall can block a significant angle of the environment. Render again. No change to the appearance of the prop.

Now duplicate the wall, so there are two of them. Render again. The wall now blocks IBL and the surface facing the wall are darker.

What is your theory?

Variations:

Turn either or both walls around, so the normals face towards the prop. Same thing happens. One alone does not block IBL, two do.

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