title: How to debug crash in default-priority dispatch queue of iOS app?
body: <p>My App, distributed as Ad-Hoc Build, crashes on some devices with crash log like this.</p>
  
  <pre><code>Thread 3 name:  Dispatch queue: com.apple.root.default-priority`
  Thread 3 Crashed:
  0   CoreFoundation                  0x3551d296 CFRelease + 14
  1   MyApp                           0x00145dae 0xdf000 + 421294
  2   MyApp                           0x00145f6a 0xdf000 + 421738
  3   MyApp                           0x0010259c 0xdf000 + 144796
  4   MyApp                           0x000e36ca 0xdf000 + 18122
  5   libdispatch.dylib             0x34461c52 _dispatch_call_block_and_release + 6
  6   libdispatch.dylib             0x34464810 _dispatch_worker_thread2 + 252
  7   libsystem_c.dylib             0x32991df4 _pthread_wqthread + 288
  8   libsystem_c.dylib             0x32991cc8 start_wqthread + 0
  </code></pre>
  
  <p>I have no idea of what clue I can get from this log. =(</p>