broken arrow

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....