I was just googling for something and got a very stupid result.
I googled for “worda wordb”. In the search result was a page with worda in the page, but wordb was only in the page because it was part of a google ad word in that page.
At first I thought google would be a little smarter about its search. The only thing I can think of is that this particular page had its ad words configured incorrectly and so google was indexing it incorrectly.
I was going to link to the page with with the google cache URL so that you could click it and see the highlighted words, but when I did I notice this at the top of the google cache header:
These terms only appear in links pointing to this page: wordb
So what I experienced is probably by design and not a goofy ad words configuration. Google is known to place pages higher in its rankings based on the link that points to the page. Ad-words is known to place ads in the list based on links pointing to the page. It was just strange for me to see what I saw.