I stumbled onto this one when searching for [business to business].
Source <TITLE>: ...Companies, Products, Trade Leads, Business Marketplace
'to' is a stopword which is ignored - no problem.
So the query then becomes [business business]. The highlighter then produces:
...Companies, Products, Trade Leads, <strong>Business</strong>
I then spent some time chasing my tail trying to reduce things down to
a small reproducible unit, and finally decided to try searching for
any duplicate [wordX wordX], and sure enough it's reproducible with
all my indexes.