When Xander parachutes into the compound, it appears he pulls his silver handle on the left ribcage, on his parachute to jettison the canopy. In reality, the silver handle is the reserve parachute deployment handle. In real life he would have pulled the cutaway handle on his right ribcage, which is the cutaway handle. Pulling the reserve handle without pulling the cutaway handle will result in a 2-canopy out situation, which would be potentially dangerous.
When Agent Toby Lee Shavers is showing Xander the kit of field equipment, a wedding ring on Toby's hand is visible in closeups of him showing Xander the bullets for the gun as well as the binoculars but is gone in other shots and continues to reappear and disappear in between frames.
When Yorgi spots the agent in the club at the films opening, he turns to Kirill saying his name and Kirill tucks his cigarette behind his ear. When Yorgi tells him "It's time to go to work", he tucks the cigarette behind his ear again.
While trying to get onto the AHAB submarine, Xander is seen standing up in the car after the roof was blown off. In the next scene, he is seen sitting down in the car, and then seen standing after that.
When Xander is chasing the bomb-boat with Yelena, they go through a village blasting through the potato carts. Both car lights are blown out by the rockets, but reappear afterwards, then disappear again.
When the scientists present Yorgi with the detonator, the rocket contains a yellow liquid at the top, and a blue liquid at the bottom. When the liquids first begin to mix, there is a shot of the rocket where the blue liquid is at the top, which goes back to being yellow in the next shot. We can tell it is the top of the rocket because of the shape of the exterior frame.
xXx is having a lunch with Yelena at the Opera restaurant. Then the mess with the sniper begins and he escapes but is captured. The place of his capture is at the foot of the hill, on which Prague Castle stands and this is on the other side of the river Vltava.
When Xander is captured by the Americans (after skateboarding down the railings on a dessert tray) he is taken into a concert hall. The exterior shot is of the Rudolfinum Concert Hall in Prague, however the interior shot is of a completely different building, the Prague National Theater (notable for it's red velvet seating). In addition, on the director commentary, the director explains how that theater (the National Theater) is where Wolfgang Amadeus Mozart premiered "Don Giovanni." This is also incorrect as that opera premiered at the Estates Theater, another theater in Prague but not pictured in xXx.
The NSA wouldn't send agents overseas - that is what the CIA is supposed to do. The NSA's work is limited to the collection and analysis of foreign communications and foreign signals intelligence; it does not perform field or human intelligence activities.
The National Security Agency is solely responsible for communications intelligence. They are tasked with securing U.S. communications and trying to intercept and decode other countries' communications. They do not have field operators of the type portrayed in the movie. If they had actionable intelligence it would be turned over to the CIA for their use. Also, the NSA is forbidden to intercept communication originating in the U.S. Exceptions require approval from DOJ and court review process and are very rare.
When XXX arrives in Prague, one of the first exterior shots is a street lined with Romanian flags.
On the street, called Nerudova, are both the Italian and the Romanian embassies. So, one of the flags is the Italian flag.
Yelena is an agent of the Russian FSB (Fedaral'naia Sluzhba Bezopasnosti - Federal Security Service). FSB is charged with internal security in Russia (like the FBI in the US or MI-5 in the UK). A Russian "agent" working outside of Russia would work for the SVR (Sluzhba Vneshney Razvedki).
When everyone is pinned-down by the sniper, Xander grabs the heat seeking rocket and says, "That son of a bitch is smoking". The rocket, if it were really heat seeking, wouldn't detect something so small as a cigarette much less turn at a near 90 degree angle to hit the guy smoking it.
During the final scenes, Gibbons orders that Ahab be fired on to destroy the weapons aboard, sacrificing one city, rather than ten. After receiving word that a car is chasing Ahab, rather than canceling the airstrike, Gibbons places himself at the epicenter of the weapons blast zone, ensuring his own death if the driver turns out not to have been Cage.
After the gentleman who was going to torture them blows up, Xander calls the other two gentleman that he was ejected from the plane with by name. At no time shown did they spend enough time together (conscious) to learn each others names.
Once all the scientists have been killed by the poisonous gas, there is a shot of a gauge indicating the amount of poison gas in the air. The gage is marked with units of kPa (Kilopascals), a unit of pressure. Hardly able to measure the lethality of the air.
Towards the end of the film, Xander's 1967 Pontiac GTO is heavily customized by his agency and receives custom exhaust sidepipes as one of the modification. However, you can still obviously see the car's tailpipes sticking out the back.