After surviving the worst of her cancer, Izzie gets fired during the Mercy West merger in season six, Alex and other doctors believing that it was too soon for her to work. Izzie leaves Seattle, believing that Alex had screwed her over. She returns in the middle of the season, much to everyone's surprise. Derek more or less promises to hire her back if he's chief (and he would be if Richard is let go), but Izzie disappears after Alex tells her that he deserves better. But who knows? Even though Katherine Heigl left the show on a not-so-subtle note, we're not ruling out Izzie's return.
We should also mention that Izzie underwent probation for cutting the LVAD wire, which is probably a fireable offense at any other hospital. Izzie gets off of it after helping a patient during the ferry disaster.
