NSMutableArray isn't thread safe anymore

gistfile1.m

NSMutableArray *resources = [NSMutableArray new];
[otherObjects enumerateObjectsWithOptions:NSEnumerationConcurrent
                               usingBlock:^(id obj, NSUInteger idx, BOOL *stop)
{
    id resource = [obj hardToCalcResource];
    @synchronized(resources) {
        [resources addObject:resource]; // Resources may be corrupted
    }
}];