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

Bad Thing
 n.

    [very common; always pronounced as if capitalized. Orig. fr. the 1930
    Sellar & Yeatman parody of British history 1066 And All That, but
    well-established among hackers in the U.S. as well.] Something that can't
    possibly result in improvement of the subject. This term is always
    capitalized, as in ?Replacing all of the DSL links with bicycle couriers
    would be a Bad Thing?. Oppose Good Thing. British correspondents confirm
    that Bad Thing and Good Thing (and prob. therefore Right Thing and 
    Wrong Thing) come from the book referenced in the etymology, which
    discusses rulers who were Good Kings but Bad Things. This has apparently
    created a mainstream idiom on the British side of the pond. It is very
    common among American hackers, but not in mainstream usage in the U.S.
    Compare Bad and Wrong.


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

Bad Thing

    (From the 1930 Sellar & Yeatman parody "1066 And All
   That") Something that can't possibly result in improvement of
   the subject.  This term is always capitalised, as in
   "Replacing all of the 9600-baud modems with bicycle couriers
   would be a Bad Thing".

   Opposite: Good Thing.

   British correspondents confirm that Bad Thing and Good
   Thing (and probably therefore Right Thing and Wrong
   Thing) come from the book referenced in the etymology, which
   discusses rulers who were Good Kings but Bad Things.  This has
   apparently created a mainstream idiom on the British side of
   the pond.

   [Jargon File]