For a small number of queries, Google shows a list of three results for other related query in the middle of the search results page. The "inline revisions" are only displayed if Google is confident that the new query provides better results.
We'll call a word or expression Google-difficult if the search results page shows an inline revision for "dictionary". That means many people who searched for that word changed the query to "dictionary" to find its meaning in a dictionary. Some examples: conceptual, tentative, salient, lucrative. Because Google's results change all the time, a word that was Google-difficult at some point will not lose its quality even if it doesn't return an inline revision for "dictionary".
Can you find other Google-difficult words? Use an English interface for Google.