Right, sorry if the pictures are a bit small, but look at the group on the right where I'm previewing a 3D effect.
Notice how there are three turquoise stripes running along it. (You can look at the 2D version)
Basically, I change the extrude depth from 50pt to 30pt and for no reason at all, the middle turquoise shape vanishes!!
Is this a bug or what?