A WebmasterWorld thread is reporting several webmasters noticing that Ask.com's crawler has recently been generating tons of 404 (file not found) errors on their sites. The issue appears to stem from Ask.com auto inserting URL-Encoded spaces into the URL. URL-encoded spaces are those %20 signs you may find in URLs.
The specific crawler called out is crawler100.ask.com. Reports say that this has been going on for two weeks now.
Forum moderator, jdMorgan, offered up a mod_rewrite technique to force the Ask.com bot to obtain the correct URL and not a %20 URL. So if you are noticing this issue as well, check out the thread and try implementing jdMorgan's solution.
Forum discussion at WebmasterWorld.