People (rightly) want to ensure proper logout from their financial site. Exploiting this flow for the ever-scarce user attention, an Indian mutual fund company, Reliance, now shows an advertisement in a lightbox when you click logout link. The logout request is processed only when you close the lightbox (say by clicking the X button on top right).
I believe this is an unethical, and a desperate technique. In addition to being obtrusive, it endangers an important step, especially if the user is accessing from a non-private computer.