Microsoft Word.

One potential customer is experiencing a severe case of content rot as their document's carefully crafted styles (used as structuring markup) are being eaten by incompabilities between versions of Word.

Of course, it's all opaque in there, no way to find any clue except by guessing.

And of course, there's no sane way to run the old version along with the new one as a workaround.

Actually the major problem is not so much Word as its closed format. Too bad someone threw away the key to their locked data!

Cocoon might well come to the rescue here. Once someone finds that key, that is ;-)