Not_obvious_EXC_BAD_ACCESS.m

-(BOOL)testItem:( id )item_
      withError:( NSError** )error_
{
   if ([item_ isEqual: @(3)])
   {
      *error_ = [[NSError alloc] initWithDomain: @"MyErrorDomain" code: 1 userInfo: nil];
      return NO;
   }
   return YES;
}

-(BOOL)processArray:(NSArray*)array
          withError:(NSError**)error
{
   [array enumerateObjectsUsingBlock:
    ^(id obj, NSUInteger idx, BOOL *stop)
    {
       [ self testItem: obj withError: error];
    }];
   return NO;
}

-(void)crashMe
{
   NSError* error = nil;
   [self processArray: @[@1, @2, @3, @4]
            withError: &error];
}

[self crashMe];