Extremely bizarre error in FrameMaker 6

[Steve Rickaby]

Unstructured, Mac. 

While editing a figure caption that consists of an in-line paragraph with the number, and a follow-on paragraph with the caption (for historical reasons), I got an error log with the following message when I try to type or edit the caption. Does anyone have a clue what it might mean or imply? 

'Layout/Pack loop exceeded 100 iterations'

and the edit is refused.

The only change here is that I've recently gone from Mac OS X 10.3.9 to OS X 10.4.5. I sincerely hope that that's not relevant: booting back into 10.3.9 shows the same behavior.
-- 
Steve