There are cases where Google doesn't return anything close to all known exact matches.
1. Most large classic forums using vbulletin. Try picking any rare word or phrase with less than 100 total matches via the forum's search tool and compare to the Google verbatim results.
2. This very site, searching for an uncommon word such as "memeplex" returns hundreds of unique results according to hn.algolia.com, but only 65 according to Google via site:news.ycombinator.com "memeplex".
3. Fanfiction sites such as fanfiction.net .
Try randomly picking an obscure 'fandom' with only a few hundred stories, and search for the name of one of the main protagonists. It will only retrieves a small fraction of all the existing stories that mention the protagonist's name.
EDIT: I originally had another example involving macrumors.com but then realized there was a formatting mistake in the search query.