Source : Free On-Line Dictionary of Computing

plaid screen
     
        [XEROX PARC] A "special effect" that occurs when certain kinds
        of {memory smash}es overwrite the control blocks or image
        memory of a bit-mapped display.  The term "salt and pepper"
        may refer to a different pattern of similar origin.  Though
        the term as coined at PARC refers to the result of an error,
        some of the {X} demos induce plaid-screen effects deliberately
        as a {display hack}.
     
        [{Jargon File}]