It took a while to figure out what pattern was afoot. At first, we attributed this to my usual sloppy typing, but eventually I started checking my original documents and discovered there was nothing wrong at my end.
And then it took a while to realize that this problem was only surfacing with people who were still using Word 2007.
The fix? Well, I don't have a perfect one. The workaround is simple, though: When sharing documents with Word 2007 users, be sure to use the older .doc format, not the new .docx. That eliminates the incompatibility issue.
Have you experienced any strange or incompatible behaviors with Office? Please sound off in the comments -- and share any fixes you know about.