int main (void)
{
    const int foo_readonly = 10;
    int *foo_ptr;

    foo_ptr = (int *)&foo_readonly; /* (1) This casts away the const qualifier */
    *foo_ptr = 20; /* This is undefined behavior */

    return 0;
}

Quoting ISO/IEC 9899:201x, section 6.7.3 §2:

If an attempt is made to modify an object defined with a const-qualified type through use of an lvalue with non-const-qualified type, the behavior is undefined. […]

1. In GCC this can throw the following warning: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]