This works at higher levels of abstraction, too. For instance, NetApp filers have a deduplication feature, where identical files are detected and stored once instead of several times. When one of the files is changed, supposedly a copy on write happens. Yet in practice, I saw, more than once, two identical files, with completely identical time stamps, owned by two different users, where only one file was modified intentionally by a program ran by one of the users (that program logged its actions to a file, and the other user's log would be empty, plus there was no chance that both modified their files at the exact same time.) I concluded that NetApp's deduplication wasn't on the beaten path - or perhaps something in the timing or other specifics of our creation and modification of identical files was unusual.