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....