WebmasterWorld Senior Member, JAB Creations, documents Ask.com's spiders crawling non-existant pages. I don't fully understand why this would happen, but typically a member at WebmasterWorld with that many posts, tend to know what they are doing. So it must be somewhat out of the ordinary.
Basically, JAB Creations feels like the search engines are crawling his sites as if they are "crawling someone else's site." The waste of bandwidth and CPU because of these spiders can be upsetting to any webmaster.