During the filming of the scene in which Tequila is running down the exploding hallway with the baby in his arms and explosions at his back was shot twice as John Woo wasn't happy with the first take -the explosions were too far behind Yun-Fat Chow. For the second take, he took control of the explosives button, and set it off far closer than Chow was expecting. "He was really running for his life." Chow apparently was professional enough to ask how it looked after the shot was finished, "but then he turns around and says, 'that motherfucker.'"
Using the shotgun in the rose box was an original idea in both this film and Terminator 2: Judgment Day (1991). It is a coincidence that they both came up with it at the same time. Its appearance in this film is not a reference to or a copy of "Terminator 2". It was used in two influential earlier films: Dog Day Afternoon (1975) when Al Pacino's character brings a rifle into the bank, and before that in Stanley Kubrick's film noir classic The Killing (1956), when the gang smuggles their heist gun into the track locker room hidden in a box of roses.
John Woo had previously been criticized for glamorizing gangsters in his films, so he decided to make this film glamorizing the police.
The teahouse where the first sequence was filmed was demolished five days after John Woo was done. During filming, the neighbours called the police every night to complain about the gunfire, but the cops were fans of Woo, so they allowed him to complete shooting every night.
Because of the shooting schedule, the crew only had one chance to perform the continuous take during the gun-battle in the hospital.