Source: WowAce (PM)

I've been searching online and in the options menus for a way to display just one progress bar, as the second often shows items that are irrelevant to what I'm grinding at the time, but I haven't found any way to do so. Is there some way I could do this?

I've seen the second bar track the K'uddly pet from the Forgotten in the Vale of Eternal Blossoms, as well as the Golden Hearthstone Card from Gul'dan. Those respectively appeared (or remained from earlier) when grinding a Captured Firefly from Zangarmarsh, and a Great Sea Ray from Dazar'alor.
It's possible that this issue is simply an addon conflict, causing the second bar to behave unpredictably. Logging out and in clears the second bar, but I'll need to test further to see how it happens and whether disabling other addons will fix it.
If it helps, I typically had the bars hidden, and the two mismatched bars appeared when I clicked the minimap icon to show them.