Just saw something I never noticed before at Google, when using the Firefox search toolbar box. (The search box that floats to the top-right of the main address bar)
1 - Type a search string into the search box, press enter or click on the magnifying glass. The results appear quickly as expected, as do the top and sidebar ads.
2 - Click on the same magnifying glass again or just press enter. (Don't change the query at all) The organic results flux the top 2-5 results, and the Google ads all refresh to different ads. Completely different ads... almost like they are being cycled. Top 5 organics flushing around as well.
This must be some kind of test to measure CTR of both ads and organics.
Edited to add: I just ran through several commercial queries. Some of the ads are cycling through different versions of the same ad. Others are jumping around to sidebar, etc. A complete jumble of the ad listings.
Also note that shopping results and sitelinks are jumping in and out on refresh of the organics.