There really is no such thing as a force field, even in james bond movies. They may have said that only to cover the mistake. If they had forcefields, not only would they have mentioned it. it would be a large plot device.
OK, lets get a few things straight.
1. When the bad guys where trying to get into the car, they only shot the side panels, not the window. They only use a machine gun for this. When they shoot it from the distance, they use multiple machine guns and a slug thrower on the window which is what caused it to break after it was weakened.
2. There was no force field on the car, just very tough armor. The electricity you are thinking of was somehow connected to the door handles. This made it impossible to grab the door handle to open it.
This is definitely not a slip-up. Being a great Bond fan, I have come to notice many little surprises that Q has put into his machines that have not actually been mentioned in the movie. This is one of them. He made it so that when the car is not being driven and has had the security system turned on, the car is impenetrable. However, when the car is in use, it is normal. If I can remember correctly (I haven't seen it for a while), this was done through electric currents and one of the henchmen actually got electrocuted (again, I'm not sure). I think I can even remember Q mentioning this quickly.
Think about it... If the car was traveling fast towards the guns then there would be the speed of the car PLUS the speed of the bullets. Secondly, There was somekind of forcefield around the car as when Carvers men try to touch it they get an electric shock
This is the real reason! If you watch it properly when Carvers henchmen are shooting at Bond's BMW they have M-16 Assault Rifles with grenade launchers underneath. When they are shooting, there is a close-up view of the henchmen firing and one of them fires a grenade (although this is not really visible, but it actually happens) then the front windscreen smashes. The windows are bullet-proof but not explosion-proof. A True Bond Fan would have figured this one out already.
If it was the m-203 g. launcher that shattered the glass, the grenade would go right through the glass, explode inside the car and kill everyone inside(Bond, James Bond) :-p
This does not deserve a 10.0 rating. When the car was parked at the Hotel. Bond turned the alarm on and the so-called force field stopped the bullets. However when the Windows shattered Bond was driving the car and to get in it he turned off the force field.
This was picked up in an interview.