Boolean Weirdness

[bford903]

Using Rhino/Matrix, when I make my 3D jewelry models I'll make all of the pieces (halo, prongs, stones, shank, etc.) and Boolean Union the entire model into one, air-tight polysurface for printing (closed shell, no naked edges). Sometimes during the Boolean Union process a new surface appears that clearly does not belong, usually going from one side of the model to the other. Rhino/Matrix reports the model as air-tight, however the printer does not and prints it hollow. People I've asked about it refer to it as a phantom surface. See pics below:







From some angles they're not even visible. This is by no means a common occurrence but it does happen from time to time and I can't help but wonder what going on. Wondering if it's a glitch in the Matrix (pun!) or if it's something i'm doing that can be corrected.