Okay, how many times have you tried to use Google's cache to retrieve something not at Archive.org, only to be told "Your search - cache:example.com/?id=135 - did not match any documents." Particularly annoying when it's a page you know for a solemn fact is in their cache. Turns out that you need to search for some key words. Well any key words. Search for "fhqwhgads", for all Google cares. (related, though I discovered this independently)
When you do that, suddenly the page will appear from the cache, and "These terms only appear in links pointing to this page: fhqwhgads" will be at the top of the page. Thanks to this, the Google cache just became that much more useful (since there are many sites — especially PHP sites with "?"s and "&"s in them) that get skipped by the Wayback Machine.) Hooray for Google!*
* Though I still stand firmly against gMail.