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A COBOL statement so dangerous I can hardly bring myself to describe it. It's a dirty job, but someone's gotta do it, I guess.

The ALTER statement allows a programmer to amend the target of a labelled GOTO statement. That is,

    PERFORM PARA-0200.
    PERFORM PARA-0100.
    EXIT.  

PARA-0100.
    GOTO.

PARA-0200.
    ALTER PARA-0100 TO PROCEED TO
        PARA-1050, PARA-2050, PARA-1057, PARA-3050, 
        PARA-3000, PARA-4000, PARA-4050, PARA-2050
    DEPENDING ON REC-TYPE.
    EXIT.
I guess, somewhere, there's a COBOL programmer who will describe the behaviour that results from the above as polymorphic... One such statement in a program is bad. In my experience, however, it becomes endemic, something to be used at every opportunity.

Goes hand in hand with excess use of periods.