Originally, Jurassic Park was going to end just with Grant saving the kids from the raptors. He was going to shoot one dead and then use a mechanical T-Rex skeleton in the foyer to kill the other. However, Spielberg realised that he needed one last triumphant return for the T-Rex and so changed the ending to include him in. Maybe this late change explains the long-debated question of how the T-Rex seems to appear out of nowhere?