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....