I might be exaggerating just a bit in the title of this post, but it appears that a teacher was searching for public transit information in class and was shown pornographic web sites instead. A Google Web Search Help thread says he/she searched for [is the tube free] in Google, while in class, and was shown a listing of pornographic sites.
The searcher said:
I don't even see the relevance of porn when asking a question about public transport. I know the tube isn't free, I was just curious if anyone said it was, stupidly.
Tube in this case was meant to be public transit, as in tunnels and roads like that. Not as in YouTube or TV or free television.
Most of the search results for that query lead to pornographic sites, here is a screen shot of the first two results:
Yes, my safesearch is set to the normal setting. There is no response from a Googler yet, but I see why these types of results came up for such a query - just in hindsight, not sure if they should have.
Forum discussion at Google Web Search Help.