The "antiquity" of the wallpaper makes for a curious footprint at one point.  On the clock-side, just before you enter the clock hall, there's a strip between the last door and the clock hall entrance where the wallpaper is badly stained, I suppose by proximity to a heating vent at some time in the Mansion's history.  Apparently they relocated the vent, since the stain doesn't seem to have gotten any worse over time, but the stain is still there.

image

The stain on the wall reveals an oval shape at one point, visible below one of the "family portraits" that currently hang there.

image

What is it?  That outline is from the oval-shaped portrait that hung there originally.  You can barely glimpse it in the Sandy Duncan special from 1974:

image