Subject: Some "quotes" truncated
Got it! It's when quotes are used in a subject. The first time you preview it, it looks good, but then when you opt to continue to edit your post before submitting it, the part after the first quote gets truncated. But if you fail to notice that and fix the subject, the truncated subject remains.

Note that I fixed my subject after the preview truncated it, which is why it shows in the final post.