It looked fine and suddenly…
The top view has much of the surface missing and I can see right through the solid:
From the bottom it looks like this:
Can I fix this without major surgery?
Polysurface glitching.3dm (724.9 KB)
Note: It’s not taking as long as I thought to fix. Just worried about this happening again when I get too far along to fix it.
use ExtractBadSrf then hide the rest and see for yourself what you have done to this poor surface 
specifically to this area, no idea how you managed to make that from one surface, you must have pulled some subobject edges down.while duplicating, but that would create a new surface i think, but i am not sure 
generally be aware that they are not very precise, if you draw a curve between the two ends of the gap you notice that they do not align, the same with boardering curves, that can happen in models when drawn from some scanned planes etc. but also to anybody drawing cad… nevermind you have to decide which precision is here needed and you want to pursue.
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Thank-you encephalon. I figured out specifically what messed up the surface. I think you’re right in saying that it’s already a bad surface. I’m trying to find a way to model this any not have things get too too ugly. I’m training to model the drainage slopes in a slab. It doesn’t have to be 100% accurate (in this case) but the slopes have to be represented. I cut a section in a different area to illustrate my intent (in case you’re curious how that mess came to be about 
):
I would have avoided this situation if I modelled the slab closer to how it would actually have been poured in real life. I now have an acceptable solid to work with, but if I run my favorite command, MergeAllCoPlanarFaces, it messes up the solid similar to the one I posed earlier. I think as long as I don’t run that command.
Pre - MergeAllCoplanarFaces.3dm (803.6 KB)
That is repeatable, so this is a good model for some bug-hunting (@pascal). Although on a casual inspection I’m not sure there are any coplanar surfaces - well, not with the one that goes bananas.
Hello- I think the quick n easy way to clean up either of the models is to
DupFaceBorder, window around the whole object, then PlanarSrf from the resulting curves and join all the surfaces together, delete the original and the curves.
-Pascal