Would the Favicon update immediately? I feel like Favicons always tend to hang around for ages and just assumed they were stuck in a local memory cache thingie or it was somehow tied to the browser. I've had some browsers hang on to an old favicon for months after the fact, even if I know for a fact that the favicon was changed from seeing it on another browser.
Yes. The old image would load if it was in your browsers cache, but you can clear this by pressing Ctrl+F5.
Or try incognito.
Or try another browser that wouldn't have twitters favicon in cache
Edit: you can also check the source code and Ctrl+f for ".ico", which is the file extension for favicons. You'll find a link to our little blue bird still in the source.