Today I discovered that a number of documents I hadn’t looked at in years were no longer readable.
They were written using Circus Ponies Notebook. Trying to open them resulted in a fairly useless error saying each page was corrupt.
I opened the package and discovered that the actual storage format was Apple’s plist. Unfortunately, it’s Apple’s binary plist format, introduced in OS X 10.4.
The same problem has occurred with a bunch of files, so it looks as if at some point, a non-back-compatible change was made in the plist format. If anyone knows how to convert the old format to the new, I’d be interested to hear about it. (Apple’s plutil won’t touch them.)