This might not be a big problem, but say the following URL is a good Stack Overflow question:
Try this. Open a search engine, say Bing (I will explain later why I'm choosing Bing) and search for:
what do you use to keep notes as a developer
The Bing result will have a URL of:
The first link on the results page is that question from Stack Overflow.
Now, try searching for:
ben miller likes bananas
The Bing result URL will be:
Ben is a friend of mine who actually hates bananas.
And look what the first result is: a link to the same question on Stack Overflow.
As you can see, you can basically use Stack Overflow to manipulate URLs to get your keywords in the first results of search engines. You might wonder what the point is in that.
Say I own a commercial product. I find a relevant question on SO and create a URL with my product's name in it. I feed that URL to search engines, and post it in an answer to the question, to boost interest in my product. I guess that's not a good example, but I hope you get my point.
Another scenario: there's a good SO question about IDE x at
stackoverflow.com/questions/1234567/how-to-do-something-in-ide-x. I'm a developer for IDE y. I could post links all over the Internet to
stackoverflow.com/questions/1234567/cheap-viagra to hurt my competitor.
I think the site shouldn't return HTTP status 200 (OK) if a URL is incorrect. Of course, the meaning of "incorrect" can be interpreted differently in different cases.
I forgot to say why my example used Bing.
Google is better at dealing with these types of links. But I managed to keep the bananas URL in Google for a couple of weeks, too.
Yahoo! seems to be as vulnerable as Bing to these types of "attacks."
One more thought: What will happen when Google and other SE will see millions of links to SO that will contain uncensored words and bestiality?