Source : Free On-Line Dictionary of Computing

smoking clover
     
        [ITS] A {display hack} originally due to Bill Gosper.  Many
        convergent lines are drawn on a colour monitor in {AOS} mode
        (so that every pixel struck has its colour incremented).  The
        lines all have one endpoint in the middle of the screen; the
        other endpoints are spaced one pixel apart around the
        perimeter of a large square.  The colour map is then
        repeatedly rotated.  This results in a striking, rainbow-hued,
        shimmering four-leaf clover.  Gosper joked about keeping it
        hidden from the FDA (the US's Food and Drug Administration)
        lest its hallucinogenic properties cause it to be banned.