In my limited experience it starts getting weirder the more complicated the edge intersections get.
Here’s the simplest example I’ve had break _OffsetSubD so far:
I think the problem condition is the X intersection between two holes.
Offsetting them, with the “solid” option does this:
Note that there are holes in this SubD, but whatever’s going wrong has also broken my pink naked edge color. The properties panel says it’s an open SubD, but I can’t use _Fill on anything that appears to be an opening.
If I offset this object without picking the “solid” option, I am able to get what one would expect to be the result. and I’m able to bridge the two SubDs into one:
Though at the X intersection holes, when I bridge with the crease option, there’s one crease on the inside the X that happens when it shouldn’t.
Obviously, as with offsetting polysurfaces in Rhino, I don’t expect perfect results every time. But this is a very simple condition that maybe can be addressed. The bridging quirk is really minor, but the inability to do a solid offset with a usable result when adjacent holes have an X intersection seems like something worth fixing.
If anyone at McNeel is curious, here’s the file: These Pants Break OffsetSubD.3dm (233.4 KB)