Over the past month I've noticed some strange 404's in my Google webmaster toolbox. They are all referencing spammy pages that don't exist on my site (obviously).
A little background on the site:
About 2 months ago we 301'd several other websites to various pages within our main website. All of these sites were various companies that our parent company had acquired over the past few years. While we were at it, upper management decided to change the URL of our main corporate site.
If you recently 301-ed other domains to your central site, this may be URLs of old pages which existed on one of the other domains in the past and which still have links pointing to them from other sites. People may occasionally follow these links.
You probably now return a 404 when these URLs are requested. You may think about finding a matching page on your current site and do a direct 301 from these strange URLs to your existing page. In that case you can preserve the link juice these links may be carrying and the user experience for the visitors will be better then when they are sent to a general 404 error page.