Why is Google Webmaster Tools showing 404 Page Not Found Errors for web pages that don't have anything to do with my site?

[Pete4]

I am currently working on a small site with approx 50 web pages.  In the crawl error section in WMT Google has highlighted over 10,000 page not found errors for pages that have nothing to do with my site.  Anyone come across this before?

[MonicaOConnor]

These extensions look like they are attachments. Go into GWT, click on the 404 link, then a box with pop up, click on the  "Linked From" tab.. Go to the page and Ctrl U to see the source code. Do a CTRL F and search for the broken link. When you find it in your source code you should be able to figure out what's triggering that response. If you can't find the URLs in your source code, mark them as fixed and it should take care of the problem. Especially if they are older. It looks like it could be a shipment status, a product out of stock message, and a PDF of train schedules.

I would check the linked from pages and make sure that there isn't some erroneous code that is creating a page when it doesn't need to.

[paints-n-design]

I think you could have is the following problem:

You got a Domain wich earned Links bevore and these Links are still out there. Google can see them and you will again and again see them in WMT. 
You cant just say solved and than they are gone, caused by the backlinks. They come again. 
Check that in WMT - I dont know how it is called in english versions (I just see the german), you can click on the 404 and than take a look at "what is linking to that page"
An easy solution in that case may be to disallow "/fs" for bots. (if you dont use fs in your url structure)

[Pete4]

The 404s start with the correct url for the site but are then suffixed after the forward slash with urls such as;

fs/201410/a_Royal_Mail_Item_Is_Currently_Being_Processed_For_Delivery_.html

/fs/201410/a_When_will_John_lewis_restock_the_Dr_Dre_beats_solo_hd_.html

fs/201410/a_Food_stalls_at_train_stations_in_London_.html

There are 10,000+ of these in my WMT account.  I have never seen this before - any ideas?

[JVRudnick]

Need more data to help, eh!

[Hutch42]

What do you mean "pages that have nothing to do with my site" are these not on your domain or are they on your domain but you are not familiar with them?