Here's what I found: A. I cut back via UNDO until I could get the PDF to format correctly.

B. Then I hit REDO once. C. I saw that the last UNDO eliminated an equation that had the [PARAGRAPH] identifier beside it (on LHS). D. I thought: Problem solved. But question why it occurred? I often cut some math from a text line and paste it into an equation; eg, when I introduce math terms in the paragraph before the equation. E: So I hit UNDO again; checked PDF formats OK; then began entering new text, etc. F: 20 minutes later, PDF will not format. G: Repeated procedure from A-C above and found that I had to UNDO to an OLDER equation with [PARAGRAPH] identifier beside it (on LHS). H. So did E again. Problem is now fixed! I. Interesting behaviour: that the older PARAGRAPH] [EQUATION] took time to trigger? J. Working OK now, EXCEPT: Maybe my latest BUG report is related; FIND and CHANGE in MATH mode triggers BUG-report. Huge report (too big to copy) sent to Apple via the Mac system; so I hope it can be recovered from them. I was putting hats ^ on some vectors.