InDesign CS4 adventures: disappearing text

While editing a book, I regularly export my InDesign files to PDF, for my collegue who does the proof-reading. Today, in one of these PDFs, I noticed something odd and disturbing:

It should be the following:

What was wrong? Even in InDesign CS4 (6.0.4) I could not see anything:

Upon reflection, the problem seemed to start with the "ff" in "offers", which is a ligature, so maybe there was a problem with it. Thus I deleted the first "f", and saw that there had actually been a so-called discretionary hyphen between the two letters, but which was not visible even when hidden characters are shown, precisely because of the ligatures. Only when I turned the ligatures off, the hyphen became visible.

Needless to say, after deleting the hyphen, everything was OK and I obtained the result already shown above. In my view, InDesign's behaviour in this case should be considered as a bug. Anyway, I decided to delete all discretionary hyphens from the document, as they were simply leftovers from the original Word file (should have done that earlier, actually).