I've got two boxes with rounded corners that need to nest tightly. The rounded corners are created in ID (Corner Options, all corners - rounded - 3mm) and do not match, and I'm thinking they should? One's an inside corner, one's outside. No stroke.
They're both transparent and will interact with a background, so they should not overlap.
I understand there are ways to work around this - but is this right?
Unfortunatley this is what happens in CS5 and later with the new live corner effects. You also can no longer creat a semi-circular end on a rectangle using corners with a radius of half the rectangle dimension.
You might be able to achieve what you want with the corner options script to make the outside corners on the smaller rectangle, then copy it and use the Pathfinder to cut that shape from the larger rectangle. For that matter the pathfinder might even work with "live corner" shapes, but using the script instead will get you true quarter-circle arcs rather than the slighly lopsided ones from the live effect.
However, going on the assumption that they'd all be wrong in a mathematically idential way, I've found that flipping the inside shape along either axis makes the shape fit perfectly.
It's 2012, and never mind the flying cars we were promised, we don't even have good rounded corners yet!