If your document has uppercase headings, check they are not going to be overlooked by spell check (which happened to me this week … eek!).
Here’s an example (strangely close to my real-life experience, but words have been changed to protect the innocent).
Settings are found in the spelling and grammar dialogue box under options.
And here’s the culprit.
As you probably know, spell check is a handy but fickle friend – useful for a final sweep but not to be totally relied upon, especially if the settings are not quite right.