Source : Free On-Line Dictionary of Computing

software rot
     
         The tendency of software that has not been used
        in a while to fail; such failure may be semi-humorously
        ascribed to {bit rot}.  More commonly, "software rot" strikes
        when a program's assumptions become out of date.  If the
        design was insufficiently {robust}, this may cause it to fail
        in mysterious ways.
     
        For example, owing to shortsightedness in the design of some
        COBOL programs, many would have succumbed to software rot when
        their 2-digit year counters wrapped around at the beginning of
        the year 2000.  A related incident made the news in 1990, when
        a gentleman born in 1889 applied for a driver's licence
        renewal in Raleigh, North Carolina.  The system refused to
        issue the card, probably because with 2-digit years the ages
        101 and 1 cannot be distinguished.
     
        Historical note: Software rot in an even funnier sense than
        the mythical one was a real problem on early research
        computers (e.g. the {R1}; see {grind crank}).  If a program
        that depended on a peculiar instruction hadn't been run in
        quite a while, the user might discover that the {opcodes} no
        longer did the same things they once did.  ("Hey, so-and-so
        needs an instruction to do such-and-such.  We can {snarf} this
        opcode, right?  No one uses it.")
     
        Another classic example of this sprang from the time an {MIT}
        hacker found a simple way to double the speed of the
        unconditional jump instruction on a {PDP-6}, so he patched the
        hardware.  Unfortunately, this broke some fragile timing
        software in a music-playing program, throwing its output out
        of tune.  This was fixed by adding a defensive initialisation
        routine to compare the speed of a timing loop with the
        real-time clock; in other words, it figured out how fast the
        PDP-6 was that day, and corrected appropriately.
     
        [{Jargon File}]
     
        (2002-02-22)