It may be that runtime exceptions are caught in background threads. The @catch could be in your code or a third-party library you're using or in framework code.
On MacOS X exceptions are caught by the event loop so they don't kill your app. As we all know on iOS in the main thread there is no default @catch block for runtime exceptions. I'm not sure if there is an @catch block in a thread however. I have seen comments like yours though.