Source : Free On-Line Dictionary of Computing

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}.