Ok, so after I answered a post about this in the USM update thread, I continued to keep digging into the issue, especially after I wasn't able to replicate it anymore. I'm not a believer in the "no news is good news" type of thinking, and even though the problem went away for me, I couldn't explain WHY the problem went away.
Here's what I found.
Basically, the way geoshells are supposed to work is they fully duplicate the entire mesh of the object they are linked to, this includes any geografts attached to the parent object. This new mesh floats just outside of the parent mesh, and that "float" is controlled by the offset parameter. In a perfect world, if you set a geoshell to a .001 offset, that would mean every point on the mesh would be exactly .001cm outside the parent mesh.
This is not a perfect world. 
When you see that weird, glassy blob form on your characters, it's caused by the shell crossing the parent mesh. This can also occur when 2 meshes that are stacked on a character cross with each other.
But wait, this isn't supposed to happen, right? .001 means .001, all over the geometry of the figure!
The problem comes in when you're working with these really deep, fine distance changes, the software has trouble keeping up with all these distances. It's already calculating all the mesh of the figure, if you're using lvl 2 or 3 subdivision, you're multiplying the number of points on that mesh, AND because the geoshell is just a mimic of the parent mesh, it must match that subdivision which multiplies the points on the geoshell as well. Eventually the render engine starts making mistakes in the distances of the geoshell mesh, which gives you that mesh crossing (glassy blobs). These mistakes are very tiny, but when you're talking about these very small offsets, it doesn't take much to cause a mesh meant to float above, to partially cross underneath.
Also, because these meshes are breaking randomly, sometimes you won't see any mesh crossing at offsets as low as .0005, but this is because the render engine caught the mesh crossing, this time, and may not catch it again in another render. This explains why you may not see the error in a render one day, then you save that scene and come back to it a day later and suddenly the error appears.
Solution
I'm not a programmer, so I have to figure these issues out through trial and error, but I can say through testing that these shell meshes need to stack at minimum of .0035 above the parent object to ensure no mesh crossing, and then an additional minimum of .005 between stacked shells.
Technically, .001 works fine, in the T pose, but when you pose the figure mesh crossing will appear.
For example, if the first level shell, let's say Body Blush, is at .0045, the next level shell (example Breastacualr Areola Shell) needs to be at adjusted to .0095, and then we want to make the figure wet, so we have to adjust the USM wet shell up to .0105. Ideally these would be .005, .01 and .015, possibly a little higher if you're using HD figures at subdivision 3 or higher. On something like the wet shell however, you don't want it TOO far offset or it can create "float shadows" that make drops look like they're not actually on the skin, which breaks the effect. For this reason, you want to avoid stacking too many shells.
I'm working on a few projects, Body Blush update being one of those, and in the process I'm going to make a simple, freebie "Shell Stacker" product that will just be some 1-click shell offset settings that "should" work on any shell. While this is something most users can easily adjust for themselves in the parameters tab, there's no harm in just making a quick freebie people can use on the fly. ;)
Another example of shell conflict is in the beautiful "Nylon Stockings for Genesis 3 and 8 Females" product at DAZ. I use this all the time, but a recent update has changed the offset of the "Glossy Shell" to .001, which is WAY too close, and causes the shell to regularly glitch into the stocking. Changing this offset to .005 or higher fixes the problem. :)
So there it is. Most people probably haven't even encountered this, because how often do you really stack a bunch of shells? But when it happens and you don't understand why it's happening it's really frustrating and causes hours of tinkering to figure out what the heck is going on. :)