[Benjamino5]

Problem solved: it was the printer drivers.

The drivers for the Xerox WorkCentre Pro 65 use [i]slightly[I/] less
linespacing than the HP printers we have.

Most groups print only to HP printers, but we have two Xerox
photocopier/printer combos, and whenever the default printer was set to
those, the linespacing was a teeny bit bigger.

Over the course of a long file, when conditions were just right, paragraphs
would jump back from one page to the previous one.

I've recommended that everyone switch their default printer either to an HP
model or, as Suzanne mentioned, to the virtual Adobe PDF printer driver.

Thanks for all the tips!