Let's consider this function:

  char* strappend(char const* input, size_t size) {
    char* ptr = malloc(size + 2);
    if (!ptr) return 0;
    memcpy(ptr, input, size);
    ptr[size] = 'a';
    ptr[size + 1] = 'b';
    return ptr;
  }

This function is undefined if size is SIZE_T_MAX.

Many pieces of code have these sorts of "bugs", but in practice no one cares, because the input required, while theoretically possible, physically is not.