Do you use Dreamweaver? Have you ever been editing a file and then suddenly you can’t save it and Dreamweaver reports the file as locked? And then if you reboot to unlock the file, you discover it has vanished?
Do you use the InDesign book feature? Have you ever saved the book and then discovered that it has vanished?
Both problems appear to occur only with CS4 running on Vista. [[Edit: Not just with Vista: It's reported in a comment below to have occurred on Windows 7. -sc]] Here are the workarounds.