A Google Web Search Help thread reports that he was testing to make sure his boss's computer network had connectivity, so he searched for  in Google and up came a porn video hosted on Google Video in the top result.
Here is a picture:
The video on the right is complete adult pornographic material - so don't click on it (unless you want porn, then click on it).
The person who spotted this was very disturbed. He said:
I was assisting our CEO's secretary today, and to test whether she had connectivity, I did a quick search on google for "123456".
She thought I was being disgusting, but I eventually convinced her it was a freak occurrence.
Can you imagine that, this woman thought this guy was playing a trick on her or something. How sad.
Forum discussion at Google Web Search Help.