Favicons don't update correctly

It looks like favicons don't always update correctly. Some observations, with e.g. simplebrowser which does handle iconChanged correctly:

• http://www.p01.org/defender_of_the_favicon/ only shows a few random frames, while it's perfectly "playable" in Chromium
• Slack doesn't show the red "dot" when there is a new unread message (reproduced by private-messaging @slackbot and then quickly navigating away)
• On Google Mail, with the "show unread count in favicon" feature turned on in "Labs" the icon seems to have the wrong count sometimes (but this is by far the worst way to reproduce it)