Air frame and engines were intended not to require testing, but one older engineer spoke up and said that the engine should be test flown on a 767. He was clearly seen as being silly and old-fashioned in his opinion, but eventually he had his way. On first test there was a compressor stall due, basically, to gravity, which had not been considered in the design software. A classic example of E-World and Real World not fully coinciding.
It was actually flown on the prototype 747 and I have never heard "gravity" as a reason for the surge before. The reason given in the film was that the engine in question had been slightly damaged in earlier severe ground testing. Even on the ground gravity acts on an engine!