It could be that they were being disrespectful to the flight crew or other passengers. Maybe they were using offensive language or being pushy when it came to things like using the restroom or getting food.
One possible reason could be that it was too edgy or different for the Cannes audience at that time. The non - linear narrative and the mix of violence and dark humor might have not sat well with some of the more traditional - minded viewers.
I'm not sure specifically which 'family booed on flight story' you are referring to. It could be that the family did something to annoy other passengers, like being too noisy or having a misbehaving child, which led to the other passengers booing them.
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There was shock among some in the film industry. 'Pulp Fiction' was a highly anticipated movie, and the booing was unexpected. It made people question whether the Cannes audience was out of touch with new and different forms of cinema.