Many Core processors: Everything You Know (about Parallel Programming) Is Wrong(my-inner-voice.blogspot.com)

  float A = 42.90F;
  float B = 00.01F;
  float C = /* ... */;
  float D = /* ... */;

  const int Iterations = 10;
  for ( int i = 0; i < Iterations; ++i )
    A += B;
  printf( "%2.4f\n", A );

  A = 42.9000F 
  B = 00.0125F

  8    iterations ==  43.0000 
  80   iterations ==  43.9001
  800  iterations ==  52.9006
  8000 iterations == 142.8820  /* we would expect 142.9 */

  float A1 = 42.9000F;
  float B1 = 00.0125F;

  float A2 = 42.9000F;
  float B2 = 00.0125F;

  const int Iterations = 4000;
  for ( int i = 0; i < Iterations; ++i )
  {
    A1 += B1;
    A2 += B2;
  }
  float A = A1 + A2 - 42.9000F;
  printf( "%2.4f\n", A );