Oh yes, favicons sometimes get messy :-/
Usually it's "fixed" when force-reloading a page (right-click on Reload button), and although only 1 page is reloaded, the favicons of all other tabs also seem to get updated, funny. Sometimes I could reproduce a mess when there are tabs with page load error for some reason, or something like that, if offline sometimes the messy icon just jumped 1 tab forward, the whole round