Among other things, I draft legal documents for a living. Sometimes Word does not highlight misspelled words with the little wavy red line as you would expect. And worse, it doesn’t report them as typos when running the spell checker.
The problem, depicted in this graphic has been the subject of numerous online discussions. See for example here, here and here. The usual forum response to this problem is "I hate Word 2007" followed by little, if any, constructive discussion of what’s going on or how to solve it.
In order to understand the solution, it is important to first understand the problem. The problem lies with the infamous hidden spelling markers that Word inserts in documents to make spell checking more efficient.