App Mis-calculated Checkmate? How is this checkmate? The King isn't even in check! Am I missing something, or did the chess app goof up?
If the app first displayed checkmate in that position, then it miscalculated.

But as Phonon noted, the app could have displayed checkmate in a previous (future) position, and then you went back a few moves. In that case it's not a miscalculation per se, but just a bug in the UI.