Memory is a bit vague but I think it was XV101 after being in the hangar undergoing mods for an extended period. On the flight test, the stick shaker went when the flaps were moved from approach to land. Gave the crew a bit of a start it did.
The system was re-calibrated and sent up for another flight. Same result.
The problem was finally traced to corrosion on the contacts of the micro-switches in the flap drum switch. Where they hadn't been used for some time, they had oxidised and affected the resistance of the circuit, so in effect, it was incorrectly calibrated.
Best bit about the stick push had to be the klaxon though.