I won’t spend a lot of time writing about this one, but I found this link (I never read the news which is why I am probably the only person who missed this) but John McCain came out on the side of gay marriage or at least that’s what his MySpace page said. Deep linking happens all the time. Sometimes it’s replaced by a goatse or tub girl, sometimes it’s targeted.
In some cases if it’s made subtle enough, it can go completely undetected, and it really mess up your site. You can force a redirection and use it to force the users to perform a CSRF attack. At the very least it causes you to have another point of failure if the resource is down. Whatever the case, it’s bad. Nuff said.