somewhere in the translation he wanted to say "水落石出". but he said "水落石...", then suddenly suffered a block, paused for a split of a second groping for the next right word to complete the idiom. but there's no time for him to think like ford did as he had to keep up with the on going translation. so he said "水落石落".
he knew that's not correct but there's nothing he could do because the right word just wouldn't come to him at that moment, probably because of the pressure of translating for the president on an occasion being broadcast live on tv with maybe billions of audience in mainland china as well as all over the world.
later he had to repeat using the idiom "水落石出" but each time he said "水落石落" instead. if chinese had been his mother tongue, that situation would never have happened.