Bug-iOS: Collection <__NSArrayM: 0x> was mutated while being enumerated.

        //init a NSArray
        NSMutableArray *arr2 = [NSMutableArray array];
        for (int i=0; i<10; ++i) {
            NSMutableDictionary *d = [NSMutableDictionary dictionaryWithObjectsAndKeys:[NSString stringWithFormat:@"%d", i], @"a", nil];
            [arr2 addObject:d];
        }
        
        //Wrong
        for (NSMutableDictionary *s in arr2) {
            if ([s[@"a"] isEqualToString:@"3"]) {
                [arr2 removeObject:s]; //while here is safe for s[@"a"] = @"eeee";
            }
        }
        
        
        //Safe 1.
        [arr2 enumerateObjectsUsingBlock:^(NSMutableDictionary *obj, NSUInteger idx, BOOL *stop) {
            if ([obj[@"a"] isEqualToString:@"3"]) {
                *stop = YES;
                [arr2 removeObject:obj]; // or obj[@"a"] = @"4ewewewe";
            }
        }];
        
        //Safe 2.
        NSArray *tmp = [NSArray arrayWithArray:arr2];
        for (NSMutableDictionary *s in tmp) {
            if ([s[@"a"] isEqualToString:@"3"]) {
                s[@"a"] = @"eeee";
                [arr2 removeObject:s];
            }
        }

PS: 搜一句补充的话

//forin 循环中的便利内容不能被改变, 是因为如果改变其便利的内容会少一个, 而系统是不会允许这个发生的所以就会crash...但是当改变最后一个的内容时, 就不会crash, 是因为此时遍历已经结束, 结束之后对内容做修改是允许的 。