Cortex-M0] I don't know why this is hard fault.

my simple code is ...
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unsigned char flag[3] = {0,};
unsigned char abc, sdf, def ;
void main (void)
{
   while(1)
  {
     //if( flag[0] == 0 && flag[1] == 0 && flag[2] == 0 )
       if ( ((*((int *)flag)) & 0xFFFFFF00) == 0 )
       {
            abc &= ~0x1;
            sdf = 0;
            if( def )
            {
                //do_something;
            }
       }
  }
}

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this code is going to hard fault handler..
and
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unsigned char flag[3] = {0,};
unsigned char abc, sdf, def ;
void main (void)
{
   while(1)
  {
     //if( flag[0] == 0 && flag[1] == 0 && flag[2] == 0 )
       if ( ((*((int *)flag)) & 0xFFFFFF00) == 0 )
       {
        //   abc &= ~0x1;
        //   sdf = 0;
            if( def )
            {
                //do_something;
            }
       }
  }
}

-------------------------------------------------------------
this code is working well..

-------------------------------------------------------------

unsigned char flag[3] = {0,};
unsigned char abc, sdf, def ;
void main (void)
{
   while(1)
  {
     //if( flag[0] == 0 && flag[1] == 0 && flag[2] == 0 )
       if ( ((*((int *)flag)) & 0xFFFFFF00) == 0 )
       {
           abc &= ~0x1;
           sdf = 0;
         /*if( def )
            {
                //do_something;
            }
         */
       }
  }
}

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this is also working well..
please let me know what's the problem.. are there any guru??