I am not sure of the time sequence the slip up is in, but I can give you the exact scence and since it only happens once in the movie all you need to do is watch the movie and you will see it in plenty of time to be ready for it.

In Mercury Rising Art Jeffries(Bruce Willis)is a fan of downers(a "street" catergory of pills. Past the halfway point in the movie "Art" is driving in his car, takes up the pill bottle looks at it, pops the top off the bottle, pours the pills out the window of his MOVING vehicle. After he pours the pills out he drops the pill bottle itself out the window. when "Art" does this you can hear the pill bottle hit the ground bounce and hit a second time very clearly(the sound doesn't fade off at all). Now if his car was actually moving(and in a city where there is plenty of noise you would not hear the pill bottle hit the ground bounce and hit a second time.)I have tried this in real life and even going as slow as 5 miles an hour I heard the pill bottle (i think)hit the first time but not a bounce and hit a second time(and it did bounce and hit a second time..my wife was watching) Now in the movie he is going faster then 5 miles an hour and at 5 miles an hour I only think I heard the first hit..but I'm not sure if thats what I heard or if it was something else (background noise from surrounding city)and even at that it was much fainter then in the movie and he was driving faster and in a bigger city then I. So they missed that in post...should have covered up the pill bottle sound or he should have been parked when he dropped the bottle. Either way it can not be replicated in real life when done like in the movie..you much drive much slower just to hear the first hit even faintly. Sounds like a slip up to me.