This is a good example. Let me flesh it out a bit more to illustrate a specific instance of this problem:

  int a[2][2];
  int f (int i, int j)
   {
       int t = a[1][j];
       a[0][i] = 0;          // cannot change a[1]
       return a[1][j] - t;   // can be folded to zero
   }

The language says that elements of the matrix a must only be accessed by indices that are valid for each bound, so compilers can and some do optimize code based on that requirement (see https://godbolt.org/z/spSF8e).

But when a program breaks that requirement (say, by calling f(2, 0)) the function will likely return an unexpected value.