/brō-kən {er-(ˌ)ō, ˈa-(ˌ)rō}/
[IBM] n. 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. In true luser fashion, the original documentation of these codes (visible on every 3270 terminal, and necessary for debugging network problems) was confined to an IBM customer engineering manual.
Note: to appreciate this term fully, it helps to know that 'broken arrow' is also military jargon for an accident involving nuclear weapons...