When someone clicks a link that doesn't exist, sometimes they get an offer to buy that website. That's always irritating. Or sometimes they get a default page. It's called a 404 error page. They can be customized, but most sites don't know or don't bother.
I've always liked mine. I figure most of the time people get there, it's my fault because some link is broken, so I use it to get messages from readers. But sometimes readers send me a broken link without saying which page they were on. That doesn't help me.
On another blog there is a great collection of artsy pages. Some make me sad for insulting the person who landed there. It's probably the webmaster's fault they're there. One insults the reader for having guessed, while mine compliments them on having guessed (because I'm trying to make mine guessable, for the most part).
Here's one that's very pretty, but you need to put the mouseover on it. I got it from the comments of the blog post I'm going to list in a minute.
(Move your mouse around it a little.)
Here's the collection at noupe.com