From the time the terrorist activates the bomb and starts the countdown from 05:00 to the time we see the bomb in the back of the helicopter 1:20 of screenplay has been shown but the counter has only counted down to 3:58.
When the bomb is armed the CIA agent tells Seagal to disarm it but he refuses, stating that tampering with it might set it off but when the bomb is seen in the back of the helicopter the printed circuit boards that acts as the electronic trigger and countdown is clearly seen lying on top of each other. This would also be extremely dangerous because the PCBs would be in danger of being short circuited and thus detonating the bomb prematurely.
When paying for the physical nuclear device with diamonds, the European-terrorists say that they will give the American thugs a key to a safe deposit box with the rest of the diamonds after they get the plutonium. More than just a key is needed to get into a safe deposit box, you have to show ID and your signature must match the Signature Card for the box. The only other way is with a court order to open the box.
The goof item below may give away important plot points.
When the nuclear device explodes, an electromagnetic pulse (EMP) would have shut down all the electrical systems in the helicopter and everything else in the immediate area. The chopper stays airborne without any problems and no lights go out in the surrounding city.