Viewer hair + shadows glitch

I was very excited today to realize that when I turned shadows on (in the release viewer) I was both seeing shadows and not seeing my alpha textures disappear! Pictures! With both hair and shadows! Squee!

But then I saw this... Noooo! What is this strange glitch around my avatar's head?

I began experimenting. It didn't appear on all my outfits. So I began taking thing off... No help there. But that shape was so familiar...

Aha! That's the shape of Ruth's hair! But why?

There's three ways to do bald hair.
1. Pull all the default hair lumps and bumps into your skull.
2. Use a transparent texture as the hair texture.
3. Wear an alpha mask that hides the default hair.

Some of my outfits use #2, and those were the ones that have the mysterious glitch. The solution is to either change your default hair to one that pulls all the hair lumps inside your head, or wear an alpha mask that hides default hair. Making an alpha mask that hides the default hair is much easier (just check the hair box when editing the mask!) so that's what I did.

The glitch does reappear when I've got the outfit in EDIT mode, but when I return to normal mode things look right again.