Example using 16-bit size_t for convenience:

  char array[60000]; // 5KB left for code and stack if not segmented
  size_t i = 40000;
  size_t j = 50000;
  size_t mid = (i+j)/2; // should be 45000
  // i+j = (size_t)90000 = 24464
  // mid = 24464/2 = 12232 != 45000

Larger integers make the necessary array size bigger, but don't change the overall issue.