There are a bunch of stackexchange "mirrors" that add no value beyond adverts. They somehow rank higher than stackexchanges in popular search engines.
And arguably even worse, they often fill the first two pages with duplicates of the first couple results, making the entire search almost pointless if those couple solutions didn't work out for you.
I've seen the same problem with craigslist mirrors. When an item on craigslist gets sold, the seller removes the ad to stop the spam. The old ads stay up on the mirrors and it's not always obvious how old the ad is for.
They often hide the fact that they mirror SO posts.
That can be very annoying, like when you're researching a rare coding issue and there are 5 Google hits, and you only find out after clicking on all of them that 4 of them are copies of the first (only genuine) one.