This happens regularly when I render in Mac Rhino.
In the lower left corner it’ll say “Rendering… 100% About 5 seconds remaining”, and in the small window on top of this it says “Render In Progress” with a Cancel button.
The “Render In Progress” window never goes away when this happens when the render is in fact complete at 100%. I always have to hit cancel for the small window to go away. This does not affect the rendering.
I cannot reproduce this at will, however it happens regularly. Not a big deal, and I thought the developers would want to know this.