What could cause the sun to appear to be missing a chunk? [Details]?

This evening on the way home I looked west and the sun appeared to be missing a large chunk. Pie chart style, with from 1/5 to ¼ missing. I kept looking at the edge as closely as possible, and it didn’t appear that something (such as a cloud) was blocking the sun. It appeared as if a chunk was missing.

I wasn’t alone, and everyone that saw it had the same impression. A chunk of the sun was missing.