Bad Javascript
Malicious javascripts can do a lot of unwanted things. I was just brushing up my javascript and suddenly realised this. Earlier, I thought the browser imposes good enough restrictions on javascripts.
As an example, the following code has made my firefox non-responsive for quite some time
<html>
<head>
<title>Bad Page, Don’t Visit</title>
<script language="javascript" type="text/javascript">
for(i=0; i<50000; i++)
{
document.write("I want to write a lot of junk to this page.<br />");
}
</script>
</head>
<body>
</body>
</html>