What is your single most effective interview question?

int main() {
    char *p = malloc(strlen("Hello"));
    strcpy(p, "Hello");
    printf("%s worldn"), p);
}

$x = array( 1 => 2 , 2=> 3 );
foreach( $x as $i => $v )
{
    print "$x $i is $vn";
}

for (i=0; i<10; i++) { printf("%dn", i); }

for (i=0; i<10; ++i) { printf("%dn", i); }