On June 4, 1997, two planes collided due to an airspace ATC error. An AirGeo B777 (Flight 1001) crashed into a Robin DR400 while descending. The Robin later hit a Stairways A320, causing the Stairways' wing to catch fire. The A320 later diverted to Ontario Intl(CA) and landed safely. There were no deaths on the A320. Meanwhile, the Robin DR400 had 5 occupants, of whom 2 survived. Unfortunately, all 397 occupants died on the aircraft. References: PSA 182 was the deadliest accident in California, Air Geo 1001 just beat it!
full title: Photograph of the 1997 Temecula Valley Mid-Air Collision Dive
