A KLM 747 attempted to take off as a Pan Am 747 was still clearing the runway in Tenerife, in the Canary Islands. Both aircraft were completely destroyed by impact and fire, with 583 dead. There were only 61 survivors. It remains the worst single air accident of all time.
The story is one often recounted in CRM training, as the KLM Captain ignored his second officer airing concerns about the Pan Am 747 not being clear of the runway before he started to take off. It is also an aviation case study in that a huge number of things went wrong in order to allow the accident to happen (from weather, to congestion - a bomb closed their original destination at Las Palmas, to the physics of VHF radio transmissions).
Here is a good summary of the incident, from the Telegraph today: