When the first-gen iPhone was out there was a TIFF vulnerability so bad that you could jailbreak an iPhone just by visiting a specific web site. I remember going to Best Buy and seeing all of the display phones had been jailbroken. (It was easy to tell - this was before the App Store, so having extra app icons on the home screen wasn't normal.)

This was a user-empowering application of the vulnerability. Obviously, a bug that allows root-level arbitrary code execution just by getting the user to load a single image could be used for some pretty bad stuff. (And perhaps was.)