Under the direction of the film's second unit, the scene that resulted in tragedy featured a small, smoking boat that was to crash into a large ancient warship. Upon collision, when machines began emitting more smoke, an unexpected flame shot up, killing the one stuntman and injuring another three (or seven) people. The exact cause of the fire is being investigated, but one report says the current theory is that "the crew might have accidentally ignited a flame that came into contact with inflammable gas on a boat, setting off the fire from which Lu was unable to escape."
I had assumed that Red Cliff was done shooting, if not because Woo was on hand at Cannes last month screening footage from the film, then because it's (according to the IMDb) scheduled to open in Southeast Asian locations in July. But I keep forgetting that the film is being distributed in two parts. The scene being shot at the time of the accident is likely for the second installment, though another report claims this was last minute addition for the installment arriving in theaters next month.
Red Cliff, which stars Tony Leung (Hard Boiled), is an adaptation of the Chinese novel Romance of the Three Kingdoms and takes place in the 3rd century, CE. At $80 million (US), it is reportedly the largest and most expensive Asian-produced film ever. According to Variety Asia, this accident was the latest of a number of problems affecting the production, most weather-related.