Revealing mistake: Several times in Venice, you can see the wake of the film crew's boat in the water. (00:11:20)
Revealing mistake: At the start, Rob and Lyle go under a bridge towards the garbage men and correctly leave a wake behind them in the water. When the shot changes to show the pursuer following them, the water is still and there's no sign of the wake from Rob's boat. (00:15:10)
Revealing mistake: When the van goes off the bridge at the start, you can see that no-one is in the van. (00:21:45)
Revealing mistake: In the chase scene in the subway, Napster stops the subway train at the end of the tunnel to block the tunnel entrance. When you see the view on his laptop, the train station CLEARLY has a metal railing. There is no such railing throughout the rest of the scene. (Yes, it's likely while filming that railing was removed to enhance the scene then put back.) (01:27:45)
Revealing mistake: As Edward Norton is finishing up the chase scene to the train station, the last turn he takes is off a one way street. But as you watch the other cars, you notice all the cars are going the wrong way down the one way street. Except for Norton that is. (01:40:35)
Revealing mistake: When Edward Norton takes Charlize Theron out to dinner, the mustache on the right side of his mouth is further away from his lip, than on the left side of his mouth showing that the mustache is only stuck on.
Revealing mistake: In the shots of the Metro tunnels, the "rails" are flush with the concrete surface, as streetcar tracks would be, to form a level surface that automobiles can easily drive on. In a dedicated right-of-way, however, the engineers would not go to this unnecessary trouble and expense; the rails would simply be mounted on the tunnel floor. Also notable is the presence of crossovers, but the absence of any movable switching equipment, rendering them useless.
Revealing mistake: When the 4x4 is arriving at the train, you can see it is not Steve driving as the driver doesn't have a mustache.
Revealing mistake: When the red and white minis are driving to the train, there are tyre marks from previous takes visible.
Revealing mistake: At the golf course, tyre marks can be seen from previous takes. They are not from golf buggies as they are the exact width of the tyres and the same distance apart.
Answer: They are flying the Italian Civil Ensign, which is used by civilians on water. The emblem in the center of the Italian flag when used on water are the Arms Of Savoy, the former royal Family of Italy.