The Jargon File (version 4.4.7, 29 Dec 2003):

software rot
 n.

    Term used to describe the tendency of software that has not been used in a
    while to lose; 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    More commonly, software rot strikes when a program's assumptions become out
    of date.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. Syn. code rot. See also link rot.

    For example, owing to endemic shortsightedness in the design of COBOL
    programs, a good number of them succumbed to software rot when their
    2-digit year counters underwent wrap around at the beginning of the year
    2000. Actually, related lossages often afflict centenarians who have to
    deal with computer software designed by unimaginative clods. One such
    incident became the focus of a minor public flap in 1990, when a gentleman
    born in 1889 applied for a driver's license renewal in Raleigh, North
    Carolina. The new 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 initialization
    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.

    Compare bit rot.


The Free On-line Dictionary of Computing (30 December 2018):

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   when a program's assumptions become out of date.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)