Source : Free On-Line Dictionary of Computing

broken arrow
     
         The error code displayed on line 25 of a {IBM
        3270} {terminal} (or a {terminal emulator} emulating a 3270)
        for various kinds of {protocol} violations and "unexpected"
        error conditions (including connection to a {down} computer).
        On a PC, simulated with "->/_", with the two centre characters
        overstruck.
     
        "Broken arrow" is also military jargon for an accident
        involving nuclear weapons.
     
        [{Jargon File}]
     
        (1995-02-07)