I was once the very junior developer on a piece of software. A customer required the latest update, but because they ran under a rather rare proprietary version of Unix, I would have to take the source files with me on a tape, and compile the program on site. Before I set out, it occurred to me from previous correspondence with them that the software licence on their compiler might well have expired. I rang them up to ask whether they had renewed the license. "Oh yes, it's still current", they replied.
This sounded just a little bit glib to me, but my bosses were happy, so off I went. On arrival at the customer site I loaded all my sources, and typed in the "make" command. An error message scrolled up the glass teletype display. The compiler's software license had indeed expired, two weeks previously.
I pointed this out to the client. While three or four people discussed the mechanics of updating the license and wondered out loud where they had kept the original installation tapes for the compiler, someone had a bright idea. He went to the server the compiler was installed on, and reset its system date to fifteen days previously.
There was a sound of several people sucking their teeth. I tried again to build my software and found, as I had expected, that the compiler didn't merely refuse to work after its license expired; it had uninstalled itself rather messily, leaving libraries and configuration files all over the place, and would require complete reinstallation once a new software license had been supplied (on a tape via surface mail).
What was immediately far worse was that the server which had just been so blithely tinkered with was also running serveral live programs. The phones started ringing continuously as users complained that they were receiving "Invalid system date" error messages as they tried to enter transactions. A few transactions did get through, with the result that several databases were now completely out of synchronization, and had to be restored from the previous night's backup, costing the customer several hours' work.
In the client's server room, the issue had resolved itself into "mutual error index" i.e. everybody was pointing the finger of blame at everybody else.
I finally completed the upgrade a month later, shortly before the clients junked their one-off flavour of Unix and went onto Linux, which would have allowed us to supply them an update on-line, within ten minutes. Ah, well ...