I might have the answer to this, Dawnseeker replied to a similar query in another thread.

Quote Originally Posted by Dawnseeker View Post
We're aware of this bug. The cause is that the client and server become desynchronized and the client believes that a star was gained when it actually failed. Refreshing corrects the display error. It's on the list of things to get fixed, but is not as of a priority as I wish it was, since it appears the game is removing your stars. I've had it happen to me a couple times as well. Its pretty annoying.