Ok, i'm talking pure software/web, i'm not on about code to power Life Support machines or NASA rockets.

In terms of software/web development what is the most dangerous single piece of code someone could put into a program (say if they had a grudge against a client/employee)

In PHP, the first thing that comes to mind is some sort of file deletion:

function EmptyDir($dir) {
$handle=opendir($dir);

while (($file = readdir($handle))!==false) {
echo "$file <br>";
@unlink($dir.'/'.$file);
}

closedir($handle);
}

EmptyDir('images'); 

Or a PHP script that takes a user's sensitive input and posts it to Google sitemap or something?

I hope this doesnt get closed off as subjective as there surely must be a ranking order of dangerous code. So i'm asking for the No.1 spot :)

DISCLAIMER: I have no grudges against anyone, just curious for the answer!