The Free On-line Dictionary of Computing (19 January 2023):

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)


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

broken arrow
 n.

    [IBM] The error code displayed on line 25 of a 3270 terminal (or a PC
    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 center characters overstruck.

    Note: to appreciate this term fully, it helps to know that “broken arrow”
    is also military jargon for an accident involving nuclear weapons....