I didn't know what was causing this problem, but Steve Lennox cleared the mystery up for me:
"It looks like an error that I have come across several times now...and it's most likely..."double sided" poly's. And/or poly's sharing the same space."
Makes sense...those ears are using a double-sided surface. Thanks, Steve!
When confronted with this problem, my workaround was using shadow mapping. In the future, I'll be more careful with my celshaded models!
You don't always want to use the default Super Cel Shader settings. Sometimes that little 0.05 difference between Zones can totally screw up that "cel-painted" look you were aiming for. If the Zones are spread out far enough, the smoothing function gives a smeared look to the "paint" that is just not attractive to my eye. Organic shapes are more likely to cause this error than flat-sided mecha, so watch out when you're animating characters. Either reduce the difference between the Zones, or eliminate their differences altogether. Don't worry about the "jaggies"--that's what LightWave's excellent Anti-Aliasing function is for!
I wouldn't call this one a problem, but you should still be aware that even if a zone is set to 0.0/0.0, it will still influence your paints. Here I set Zone 1 of all the surfaces to 0.0/0.0 with a Paint Brightness of 0. You'd think that since Zones 2-4 handle values 0.0 and up, that Zone 1 doesn't matter, right? Wrong! The unlit places on this model show up as pure black, or 0.0 Paint Brightness, even though all the other Zones are set to values of .7 or higher. I could have set the Paint Brightness to something high (since you don't technically have to choose Paint Brightnesses in ascending order...) and there'd be magenta instead of black, there. You may like this look (looks kinda cool, come to think of it), but just be aware that you have to set the Paint Brightnesses of all the Zones, no matter how many "paints" you use per surface.