Only one that I can remember in over 3.5 million miles, but a lot in long haul, so maybe a thousand landings. My delayed TWA connecting flight was about a few feet off the ground at PHL when the airport suddenly lost all electric power in a summer thunderstorm. Apparently when the lights on the runway went dark, the pilot's immediate reaction was to go back up. We returned to JFK and had a redo after waiting several hours for the weather to improve after the power at PHL came back on. When we landed, they told us that the monitors had already shown us as landed, which caused some concern in the terminal when no airplane was there.