Continuity mistake: The first four episodes of season two have absolutely no continuity with the show. Prime examples are the fact that Shendu exists again as a statue which had been confirmed to be destroyed in "The Day of the Dragon", the final episode of season one. Next during "The Warrior Incarnate" it is mentioned that not all the talismans were in fact present, such as the dog talisman, when all talismans must have been found, or the events that escalated during the final episode of season one would never of happened. This problem is the extended during the next episode where Jackie Chan discovers the snake talisman in some sort of Brazilian temple - he had already learned of the snake talisman by stealing it from the museum quite early on in season one. The final continuity error is that Tohru goes from friend to foe and then back to friend as if the first four episodes of season two never happened.
Continuity mistake: When Tohru arrives at the McDonald farm, he tosses his former Enforcer buddies into the water trough where Daolon Wong has been landed by Jackie: first Chow, then Finn, and lastly Ratso. But the threesome land in the trough in the following order: Finn, Chow, Ratso.
Factual error: When Jackie releases the lotus flower from his helmet, he starts holding his breath - an unwise move to do in reality. With the pressure in his suit dwindling rapidly and no outside pressure in vacuum to counter the air pressure in his lungs from holding his breath, he would suffer from rapid decompression. While Jackie would not explode, it would cause a most certainly fatal lung trauma.
Factual error: Jackie and Jade flee from the Magisters down a sewer manhole, starting in Wiltshire County (where the real Stonehenge is located) and ending at the bridge right next to London's House of Parliament - which lie apart by about 90 miles of linear distance. The British sure have one VERY extensive linked sewer system.
Factual error: During the final confrontation, Xiao Fung keeps blowing and blowing, and Valmont/Shendu, Finn and Ratso are blown through the tunnel and right into the middle of a garbage dump. But they have to break through a layer of garbage in the process, and since Xiao Fung's breath keeps blowing, there should have been a hole through which his windy breath would pass, instead of an intact layer of refuse.
Other mistake: When Viper slides the swords to Jackie and they come to a rest next to him, there is a small paper spider right beside Jackie. The only paper spider in the room, however, is Origami himself, and he is actually too far away at that moment and too big to fit the bill. Guess some of the animators got their depth perception messed up.
Other mistake: As we can see when Jade draws up her tattoo, it is oriented right in the center of the page and in perfect line with its sides. So when Jade applies her tattoo, the image should have been printed onto her skin diagonally, but it comes out all vertical.
Plot hole: While fleeing the guru's disciples through a narrow side street, Jackie gets cut off by the van driven by Dr. Jamba and associates, stun-sprayed in the face, and then loaded into the van. But with three brutish-looking guys being so close to their quarry, the real crooks can take the time for dragging Jackie around to the back door instead of loading him through the side doors?
Factual error: I understand that Mayans may have mined silver, but it seems rather funny that they would use European-style rails and iron ore wagons to haul the ore through the tunnels; as far as I know, it was all hand work for them. And the Spaniards, who could have introduced these techniques, would never have allowed the Mayans to create a silver statue, but would have swiped all the silver for themselves.