If you try to do path finding on a target that's not reachable the A* algorithm iterates through every possible square in the scene, which is very slow. To solve that I use floodfill to check that the target is reachable. The floodfill checks that both start and target squares are both in the same area.
Maybe you simply didn't test enough to find the bugs. I had a nasty intermittent bug in the first version of my game. It took me 8 hours of continous testing before I found it. Also, check the crash logs in iTunes connect.