HiRISE has been re-imaging regions first photographed in 2006 through 2007, six Mars years ago. This long baseline allows us to see large, rare changes as well as many smaller changes.
One of the most actively changing areas on Mars are the steep edges of the North Polar layered deposits. This image shows many new ice blocks compared to an earlier image in December 2006. An animation shows one example, where a section of ice cliff collapsed. The older image (acquired in bin-2 mode) is not as sharp as the newer one. [More at link, including animation]