Did a long render with all sorts of caustics et al turned on.  And the result was unusable.  Why?  Turns out that in C6pro, running on OS 10.4, when you check "alpha channel" in the render options you don't get one.  Instead it adds the alpha channel to the color channels.

Did a bunch of test renders and opened them in PhotoShop to investigate.  In each case, the "alpha" for the document was multiplied into the RGB channels.  It could not be selected, separated, or otherwise manipulated or removed.

And the end result was unusable.

When did this happen?  I've been rendering with alpha channel and g-buffers since, I dunno, C3.