Our company was in the business of manufacturing electric power generating systems for aircraft: generators and controllers and test sets to test the black boxes. We'd built a modular test set to test black boxes for both the Boeing 757/767 and Airbus A300-600 and A310. Then the A320 came along and we decided go to a software driven test set that could accommodate the A320 as well as the other boxes. OK, but we'd come up with a new design for the Airbus boxes and that meant you had to support the older “legacy” box testing as well as the new production units.
There was also a change in operation between “legacy” boxes and new production boxes: the generator control unit (GCU) boxes worked in the air and on the ground (no change), but the ground power control unit (GPCU) worked only on the ground for “legacy” boxes and worked in the air and on the ground for the new units.
We were running the test for the new “legacy” GPCU and it failed its test by reporting a wrong fault code. Worse, it reported several different codes. After some study and analysis, we discovered why. The old GPCU “went to sleep” when the plane went airborne (+5 volts turned off). When the plane landed, the +5 volts came back and the box “woke up”. The first thing it did was to look for an “I'm OK” pulse from the other GCU’s at regular intervals. When it didn't get the expected response, it thought there was a fault and wrote a malfunction code.
We asked ground support engineering for a fix and they quoted six months and $50K to fix the software. We had to ship the test set in a month. The solution was to simulate the “I'm OK” pulse by pushing the ENTER key on the keyboard until the right code appeared on the display. we rewrote the test directions and that's the way the test is performed today. A simple fix, but you had to understand the differences in different system operation on different generations of aircraft.