Not on BA, but I once saw a Qantas captain come down from the upper deck to explain to each cabin why the then-new 747-400 wasn't backing away from the gate. A voltage spike from the ground power had crashed all the plane's systems, including the PA and the A/C. He told us he had to re-boot the aircraft, and it would take about 20 minutes. Not really what I wanted to hear at the start of the SYD-LAX leg.