From Wikipedia:
The making of the movie had consequences that overshadowed the film itself. During the filming of a segment directed by John Landis on July 23, 1982, actor Vic Morrow and child actors Myca Dinh Le (aged 7) and Renee Shin-Yi Chen (aged 6) died in an accident involving a helicopter being used on the set. Without warning, it spun out of control and crashed, decapitating Morrow and one of the children with its blades. The remaining child was crushed to death as the helicopter crashed. Everyone inside the helicopter was unharmed.

The accident led to legal action against the filmmakers that lasted nearly a decade, and changed the regulations involving children working on movie sets at night and during special effects-heavy scenes. As a result of the accident, one second assistant director had his name removed from the credits and replaced with the pseudonymous Alan Smithee. Another result was the end of the friendship between director Landis and producer Spielberg, who himself was already angered before the accident that he violated so many codes.

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