The following CSS would cause the top-right and bottom-left corner to overlap, even without borders:

border-top-right-radius: 70%;
border-bottom-left-radius: 70%;
corner-shape: notch;

Basically anything more concave than this:

would result in the corner collapsing on each other.

Since we constraint radius to not overlap in the ordinary round case, we should probably constraint it (or constraint the shape) in this case as well.

(Brought up by @smfr)