Why does the Finder sometimes refuse to receive a drag-and-drop of a URL from a web browser?

For years I've been annoyed by how most of the time I can drag the favicon from a web browser's address bar to a window in the Finder to create a bookmark file but sometimes the Finder has a tantrum, refusing to accept any more such drags.

I can never figure out the pattern. Many macs later, many versions of Mac OS X later, and still the same occasional stubbornness by the Finder.