Search box in AdSense link units

Some webmasters have recently noticed an interesting form of advertisments: Instead of displaying a topic, AdSense shows a search box with the following label: "Displaying ads in the topics below:". I think that this type of ads are most likely displayed when Google cannot show relevant topics, similarly to "Public Service Ads" which show up occasionally in AdSense in Context units. You can see right now an example of this if you go to the following site: http://v.grocceni.com/humor.html or pick an other file from the root directory list of this server to see more similar pages. An other common feature with PSAs, that if you prevent PSAs from displaying specifying an alternate ad URL, these kind of search boxes will never appear on your page, similarly to PSAs

Sometimes Google displays a topic in link units, with no relevant ads., so when you click on that topic, a similar search box will be presented on the landing page along with the following explanation: "We are sorry, we couldn't find relevant ads to Your topic. Please type an other topic." It's a bit weird that first AdSense tells you that it has relevant ads in a certain category, and when you click on that category, no ads are delivered, only this search box. I would rather consider this as a bug, not as a feature...

As far as I can remember, it was about two months ago when I spotted this kind of ads. Since there are not so many Hungarian AdWords adwertisers, these special link units tend to appear more fequently on sites written in "smaller" AdSense languages. If there are "normal" ad units on the same page, most frequently they either show Public Service Ads or advertisements in other languages, like English, German, etc.