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

green lightning
 n.

    [IBM]

    1. Apparently random flashing streaks on the face of 3278-9 terminals while
    a new symbol set is being downloaded. This hardware bug was left
    deliberately unfixed, as some genius within IBM suggested it would let the
    user know that ?something is happening?. That, it certainly does. Later
    microprocessor-driven IBM color graphics displays were actually programmed
    to produce green lightning!

    2. [proposed] Any bug perverted into an alleged feature by adroit
    rationalization or marketing. ?Motorola calls the CISC cruft in the 88000
    architecture ?compatibility logic?, but I call it green lightning?. See
    also feature (sense 6).


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

green lightning

   [IBM] 1. Apparently random flashing streaks on the face of
   3278-9 terminals while a new symbol set is being downloaded.
   This hardware bug was left deliberately unfixed, as some
   genius within IBM suggested it would let the user know that
   "something is happening".  That, it certainly does.  Later
   microprocessor-driven IBM colour graphics displays were
   actually *programmed* to produce green lightning!

   2. [proposed] Any bug perverted into an alleged feature by
   adroit rationalisation or marketing.  "Motorola calls the CISC
   cruft in the 88000 architecture "compatibility logic", but I
   call it green lightning".  See also feature.