If you select the option "Only Show debuffs dispellable by me" and play a druid, you'll see a series of debuffs from dungeons (Normal/Heroic/TWW) showing under both the Debuffs and Raid Debuffs Indicator categories that are NOT dispellable using Improved Nature's Cure. I'm starting to collect screenshots of some of them, but either way it means you're wasting cast time trying to cure things you can't.