Google has a useful filetype search operator that helps you find non-HTML documents like Microsoft Word, PDFs or even Powerpoint presentations on the web.

Say you are searching for the operating manual of Canon EOS 5D Digital SLR Camera, you could type the following query in Google:

eos 5d site:canon.com filetype:pdf

I use this very frequently but today came across a funny bug in Google filetype operator.

Basically, if a website name ends with one of the searchable file extensions like doc or pdf, Google would confuse that website for a document. For instance, the following query will show all Microsoft Word documents stored on google.com

site:google.com filetype:doc

But if you look at the results page [screenshot above], none of the search results are actually Word documents - they are just names of Google groups that end with doc.

Update: Emma wrote to Matt Cutts about this Google filetype bug and here's the response of Matt on his blog
Emma, interesting post. I think it's known/expected that filetype:doc will return urls that end in .doc, even if the file isn't a Word file. That's why you should never name your web documents something.exe . :) I'll ask someone to be sure though in case that's new behavior.
Thanks Emma and Matt.