Stunt player Harry O'Connor was killed when he hit a pillar of the Palacky Bridge in Prague, para-sailing during one of the actions scenes. The accident occured while filming the second take of the stunt; O'Connor's first attempt was completed without incident and can be seen in the completed film.
In Colombia, Xander (Vin Diesel) jumps a motocross bike over the barn where he'd been tied up. The building explodes with careful pyrotechnics, and Xander makes it safely to the other side. This stunt had never been done before for any film.
In one scene, two cops who are heard speaking in Czech are saying, quite clearly, something along the lines of "How's it going?" - "Oh, it's cold as s**t." - "Yeah, f**king weather." The dialogue is not subtitled.
xXx's car is a 1967 Pontiac GTO. The first car purchased by the studio to build up for the film still wore its original GM/Pontiac paint color known as Montreux Blue, which, after 30-plus years in the California sun, had severely faded.
In the action scene on the drug farm, Vin Diesel and/or his stunt driver were wearing bulky helmets, which were replaced with a CG version of Diesel's head created by the people at Digital Domain, who also repainted parts of the clothing which were obscured by the helmet(s).
The motorcycle that Xander is riding after the Corvette base jump is a 1999 MV Agusta Serie ORO (Gold Series). It's one of only 300 produced worldwide. Its a 750cc, in-line 4-cylinder engine which produces 125 hp and a top speed +175 mph.
Yorgi's club in the opening sequence (with "Rammstein" concert) was actually an abandoned church north of Prague. The club where "Orbital" performs is actually a high-voltage electrical testing facility just outside of Prague.
When Xander is climbing the rocks and infiltrating the castle, he is wearing a special jacket. It's actually a textile motorcycle jacket, produced by Joe Rocket, and its called the "Phoenix" series. It uses padding and ballistic nylon weaving to reduce the impact and abrasion damage caused in a motorcycle crash.
They talk about 'silent night' as being a biological agent. It is a binary chemical agent that takes two inert items and converts them into a lethal weapon. Scientist that work on raising virus and other live pathogens don't also work on chemical agents. They are a different thing.