One of several archetypal hazards facing online web-apps. Specifically, a cautionary tale against the use of HTTP GET requests to initiate actions.
Google (and many other organizations) have automated progams (called spiders) that constantly roam the web to “see what’s out there.” They do this by following links on web pages which lead them to other web pages, ad infinitum. Normally, these spiders avoid buttons which trigger HTTP POST requests (like the submit button on a web form) but they have no compunction against following any sort of GET link (assuming that it will simply lead to more content).
If you forget this fact, and have GET links on your pages to ActionController methods that do something without sufficent sanity checks, you may find your buttons being pushed by Google.