There is no way that the scene racing up the river in Prague could have taken place as it did, as there is a weir in the way which he would not have been able to cross.
When following the Ahab, Xander and Yelena drive on left side of the river, thus they should turn right to go over the bridge. They go left.
When Xander Cage steals the red Chevrolet Corvette from the California Senator at the country club, he jams the gear shift into first gear and then continues to drive in reverse.
After Yelena shoots Milan Sova, she checks his pulse to see if he is truly dead. She does so wearing thick leather gloves, which would make it impossible to feel a pulse.
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.
Shavers claims to have graduated magna cum laude from MIT. MIT does not award Latin honors with degrees.
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.
The double swing doors in the agency showing a map of the world does not have New Zealand on it, which is quite a big land mass to miss.
Ahab is supposed to cruise the world's oceans after passing through Prague; there are dams and weirs (at least one right in the middle of Prague) which would prevent it.
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 tattoos on Xander's arm switch throughout the movie.
When XXX sneaks into Yorgi's lair to rescue Yelena, he jumps down and pushes his hand against a 'brick' wall - the wall gives noticeably indicating it is not solid.
Ahab was meant to travel to different continents, but the river through Prague flows south to north, and as Ahab was cruising south, it was heading inland.
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.
In the slow motion shot of the Corvette going off the side of the bridge, the car bursts into flames a split second before hitting the ground.
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.
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).
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.
While chasing the Ahab on the river inside the GTO, during close-ups shots "Yelena" has tooth braces in some shots and then the other close-ups her tooth braces disappear, back and forth.
When coming in for a landing after jumping from the stolen car, you can see the xXx tattoo missing from the back of his neck.
When Yelena pulls onto the bridge towards the end of the movie, the GTO is maroon rather than blue, then changes back in later shots.
Crew member in orange jacket is reflected in the window of the restaurant where XXX takes Yelena for lunch.
When Xander steals the red Corvette, the people the Senator and Janelle are talking to briefly disappear (then reappear) between shots.
The logo on the front of the Pontiac GTO disappears and reappears between shots throughout the movie.
When Xander arrives in Colombia, his jacket disappears and reappears between shots.
The black eye and broken nose of Polk, the head-butted agent, are healed only a few days later on the bridge for the climax of the movie.
When Xander is using the heat seeker rocket, he adjusts the screen on the side so he can see it. In the shot where we can see the screen, there is nothing on it.
Right after XXX gets on the Ahab, there is a brief shot of the Ahab from behind. The turbines of the Ahab's engines are not moving, but the Ahab is obviously moving very fast.
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.
When Xander stands on the mountain throwing the grenades, his jacket is open and in the next cut closed.
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 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.
In the club, the members of Rammstein have flamethrowers but one of them disappears in one shot.
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.
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.
When Xander throws the bombs that start the avalanche he opens his jacket to take out the explosives. In the next shot his jacket is closed but there is too little time to zip it up.
As Xander cuts the parachute free after landing on Ahab, a cameraman is clearly visible on the front of the boat.
When Xander and Yelena are seen in the restaurant from the sniper's perspective, a crew member is reflected in the window (wearing a baseball cap and red jacket).
When the Czech cop is sitting opposite Xander and lifts a gun to shoot him, the gun alternates between resting in his lap in long shot and raised in his hand in close-up shots.
When the "care package" takes Xander to the basement to show his tools to him, a wedding ring is visible in some shots of the "care package's" hands, but not in others.
When Xander is Sky-surfing towards the communications centre in one of the angles you can see the shadow of a helicopter on the ground below him.
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.
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.
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.
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 Xander is taken by the NSA in his apartment/loft he is not wearing a jacket, when he is in Columbia the jacket appears out of nowhere.
During the Snowboarding scene, Xander's stance changes from Goofy to Regular foot and back repeatedly between cuts.
When Xander tells Yelana that he's a secret agent, she puts her hand to her mouth and laughs, but in the next shot her hand is on the table.
During the van scene when Xander is taken, while fighting the bad guys the sliding door to the van gets torn off, but in the next scene it appears back on.
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.