I should add that this invariably happens when I don't have the time to debug things. Then nothing works: clearing cookies, going incognito, switching browsers. But of course when I go looking for examples right now everything seems ok.

I'll keep refcontrol in mind next time it happens, thanks. I'm not sure why setting the header this way would work when clicking directly on the link or the google search page doesn't, but heck I have no model for this behavior anymore.