A precision: it does not matter which word it is, and the issue is over uppercase letters. Meaning that the presence of a non-indexable word (e.g. the, who, which, about) with a single uppercase letter at any location (This, tHis, thIs, thiS, tHIS) will make the query fail, and that word can be...
Our Xenforo forum has recently migrated the search function to InnoDB. It works better in most cases, but in some odd cases will return no results when the searched term begins with a common word.
For example a thread titled "how problems with search began" (not a real example but close enough)...