Trivia: The scene with the helicopter putting the bus on the building was technically impossible as no building roof could hold that weight safely. So the crew built a set of the roof that was reinforced to hold the bus. The rest of the city was CGI inserted around it.
Trivia: Very little of the bus/helicopter scene was CGI. The director insisted that a real helicopter be used that could handle the load. This made the crew and the pilot extremely nervous as the load was almost 80% of the helicopter's capacity. The stunt crew held firm on the scene where the bus crashes into a meeting however. That was completely CGI as the pilot insisted that it was way too dangerous to attempt it even with a fake building.