@ovid@cstross That reminds me of a story someone told me many years ago, so it's not first-hand but I can certainly see how it's true.
The company that the person who told me this worked for (or consulted for) was unable to run the program they had been using for ages. This happened after the machine was rebooted (or IPL'ed as it's called in the IBM mainframe world).
When they analysed the problem they realised the program itself was no longer on the disk. It had been deleted, and since it was already loaded, no one noticed until they had to reboot which cleared memory.
They didn't have any media for it, so eventually they had to contact the vendor. Turns out the had gone out of business years (10 or more?) prior. The application had been loaded by a representative of the vendor, and the machine hadn't been rebooted since then.