title: Getting "malloc: *** error: incorrect checksum for freed object" inconsistently
body: <p>The full error is:</p>
  
  <pre><code>app(85540,0x38661a8) malloc: *** error for object 0x11214f84: 
  incorrect checksum for freed object - object was probably modified after being freed.
  </code></pre>
  
  <p>So I'm getting something that is quite hard to replicate and I suspect is has something to do with the way I have my block set up. What I'm trying to get is the current online status of gamers on Xbox Live, so I allocate 2 <code>NSMutuableDictionaries</code> in <code>viewDidLoad</code>. <code>onlinePlayers</code> is for holding the online status values of the gamers so it's not checked over and over again in <code>cellForRowAtIndexPath</code> when scrolling up and down. <code>checkedPlayers</code> is to prevent multiple calls going out trying to get the status of the same player. Anyway, if I keep launching the simulator over and over again, it will be fine 29/30 launches, but it always crashes at least once on launch with the above error when I'm trying to set the online status value for a gamer:</p>
  
  <pre><code>        NSString* gamertag = cell.gamerTagLabel.text;
      if (![_checkedPlayers containsObject:gamertag]) {
          [_checkedPlayers addObject:gamertag];
          [Utilities processJSONDataWithGamertag:gamertag andBlock:^(NSData *jsonData) {
              id onlineStatus;
              NSDictionary *allXboxAttributes = [Utilities returnJSONObject:jsonData];
              // Get current Xbox Live Data
              if ([allXboxAttributes objectForKey:@"data"]) {
                  NSDictionary *dataXboxAttributes = [allXboxAttributes objectForKey:@"data"];
                  onlineStatus = [dataXboxAttributes objectForKey:@"online"];  
  
                  // Crashes on the line below     
                  [_onlinePlayers setObject:onlineStatus forKey:gamertag];
  
              // Return to main thread and update online status
              dispatch_async(dispatch_get_main_queue(), ^{
                  [self.tableView reloadData];
              });
          }];
      }
  </code></pre>
  
  <p>If it helps for some reason, the value being returned from <code>dataXboxAttributes</code> is a <code>BOOL</code>. Any help would be much appreciated. Thanks!</p>