Support tech caught by "Technician Aura": the bug that only hides when you're watching

The mystery

It has been reported that a telco field engineer — anonymised as "Benny" — was sent to a large Australian gardening store in the 1990s to hunt down a point-of-sale fault that struck several times a day. The shop’s payment setup had a master terminal tied back to a Honeywell DPS6 at a local exchange, with other tills on an RS485 bus. Benny swapped in the backup modem, ran extended bit‑error tests, replaced the master terminal, checked the RS485 wiring, and even hooked up a protocol analyser plus differential probes to an oscilloscope. Nothing. Not a peep. Classic Technician Aura. Or as programmers call it, a Heisenbug — the thing that disappears when you stare it down.

The reveal

After hours of waiting and a growing sense of futility, Benny started writing his report. Then a colleague turned up and, casually, set his GSM phone next to the master terminal. The phone rang. The system crashed. It has been reported that the cashier normally left her mobile on the shelf beside the terminal; when Benny’s kit forced her to move stations, the interference stopped. Allegedly, the ringing handset produced enough EMI to upset the POS electronics and take the payment system down.

All that oscilloscopes and protocol analysers, and the fix was to put the phone two feet away. Relief, embarrassment, a bit of bemused triumph — that’s the emotional beat here. The story is a tidy reminder: before you rewrite firmware or replace hardware, ask the obvious question. Could the solution be as mundane as a misplaced mobile? Sometimes the simplest culprit is also the loudest.

Sources: The Register