[lwf]

It also means you can trick people:

http://www.amazon.com/Intel-Quantum-Computing-Module/dp/B001...

[nmjohn]

Amazons URL's are actually quite interesting -

    Original: 
        http://www.amazon.com/Structure-Interpretation-Computer-Programs-Engineering/dp/0262510871/

    Equivalent:
        http://www.amazon.com/dp/0262510871
        http://www.amazon.com/dp/0262510871/something-else
        http://www.amazon.com/something/dp/0262510871
        http://www.amazon.com/something/dp/0262510871/something-else

It appears so long as 'dp/0262510871' is in the url (without dp/# appearing before it, but a second one after is fine) it works.

[cpeterso]

Or simply http://amzn.com/0262510871

[rawdisk]

HTTP/1.1 301 Moved Permanently

This is a URL shortener that just redirects to the full URL that has same number. Easier to type but otherwise acomplishes nothing. Server with the content still needs full URL. All this shorter URL gets you is the full URL.

[ademarre]

You can trick people, and possibly even search engines. I've wondered if blackhat SEOs could abuse such URLs to discredit content on competitors' sites.

I believe it can be a negative signal when sites stuff too many keywords in their URLs, especially if those keywords aren't relevant to the page's content. A server accepting arbitrary URLs is in a way blindly sanctioning loaded URLs.

Granted, Google's algorithms are surely very sophisticated in this regard, but fighting web spam is hard.

[MartijnHoutman]

Surely, a correct canonical URL will prevent this from happening.