When the minion brings the whip down, for one frame it disappears and the minion is only holding the handle.
Over the time period of Jack training from a kid to a young adult, the era Jack seems to live in varies wildly. He is trained in the periods of ancient Egypt and the colliseum of ancient Rome, but then, he's training with Robin Hood (or someone of the same period-dress), who lived in the Medieval Ages, which wouldn't occur for another 600 years. He is also training with the Russian man in what appears to be the medieval period. And after that, he goes back to what appears to be the ancient times of India.
Throughout the entire series there are moments when Jack--who is supposed to be an expert samurai--holds his sword wrong. Samurai are trained to be right-handed through strict discipline. The way you properly hold a sword is with your left hand above your right (your right hand should be toward the bottom with the left closer to the top.) The animators have drawn Jack with his right hand over his left, which is incorrect.
When the minion of Aku brings his whip down on the emperor, the whip is already fully extended. But when Jack catches it, it is still behind the minion's head.
When Aku first transforms, he slashes Jack across the back. When this happens, Jack is not holding his sword. But in the next frame, there it is!
The dogs track ink across Jack's banner, left to right. However, in the next shot, the prints are down to up.
When the dogs walk across the banner, they leave their paw prints diagonal, left-right-left. However, when Jack rides out, the three prints are in a straight line.
The square within a circle on the war banner has the square tilted diagonally. However, when Jack rides out, in one shot the square is level.
Jack was cutting drones in half with his sword all through this episode, but only toward the end of the episode do the drones start spewing oil when cut in half.
During Jack's conversation with the Chritchellite after he subdues the Woolie, Jack's scabbard disappears for one shot.
When the Woolie first asked for help, there were only two Chritchellites, but when they went in to zap it, there were three.
The bounty on Jack's head is two googleplex. A googleplex is a number that is described as a one with a google zeroes after it. Aku couldn't have that much money; every creature in the universe combined would not have that much money. If you were to count every molecule of every element of everything in the universe, it would not equal two googleplex. It would not even equal a google. Simply writing out a googleplex would take more space than the universe would have room for, and would take more time than the age of the unviverse.
After Jack is attacked by countless bounty hunters his sandal breaks. In a blind rage he picks it up and hurls it at a digital wanted poster. The sandal falls to the ground and Mad Jack, who is created by Aku, grows into the sandal. However, during the fight between Jack and Mad Jack, Jack has two sandals on.
A little before Jack gets zapped by the lava monster, you see Jack sheathe his sword. When the camera view switches to a long shot, his sword is unsheathed. However, when the camera goes to another closeup, Jack's sword is sheathed again.
Although this is a very amusing episode, it features one of the worst Scottish accents I have ever heard. (editor's note: I think that was the point... :) )
The Scotsman's machine-gun peg-leg, although a creative weapon, would not work. Machine-guns like that have QUITE a kickback, and if he fired it while standing on one leg, he'd more than likely fall over (and break the other one). (Note: In this episode when he fired it, he was in the air)
Why doesn't Jack just jump right over the Scotsman, Tremendous Horsecut-style, and continue along the bridge instead of spending days fighting him?
Why didn't Jack just cut the handcuffs off with his sword?
When the fight breaks out in the club, and the woman on the stage is still singing, the band that was playing behind her is gone (along with their instruments) but the music is still playing.
When the young Monkey Boy and the other slaves are walking in the machine, they are going forward, technically making the belt turn counter-clockwise. But, as we are zoomed out, we see the spikes on it going clockwise, opposite of what they should be.