When I'm giving an error message for people trying dumb tricks sometimes I like to use this image: http://sla.ckers.org/images/idiot.jpg

I think it's subtle, but at the same time, mean spirited.

I like this one, what about an additional link to amazon ;)
But I know this stuff, such as redirects to disney.com and so on.

I usually use more general error messages, I don't want to provoke people to get deeper... You can't always be sure your site is 100% secure, I think error messages like that make people more motivated to hack your site.

talking about error messages, google's error messages are worth noting.

One of these days I'll throw up some of the more amusing error messages I get from everywhere. I used to keep a document with all of them in one place so I could go back and comment on them. I did put a few online. Like this: http://www.fthe.net/i/googleerror.jpg and http://www.fthe.net/i/gmailbanner.jpg