The shoe salesman's shop is named "Foot-Chalet." However, when Jack drops down and lands in the broken glass, the hyphen in the sign disappears in one shot.
Trivia: Among the shoes Jack tries include a pair of spiked heels (which he realizes too late that they're actually a woman's shoes); cast-iron shoes (that prove to be too heavy for Jack to jump in); wooden shoes with a water-base complete with real goldfish (which break in about 3 seconds); a simple pair of tennis shoes (that get stolen from Jack by two street punks); extra-furry boots (that are revealed to be living creatures); and a pair of experimental tennis shoes that "pump up" and give Jack extra jumping power (although the pumping part pops and Jack goes flying).
As the enraged Demongo releases all of the warriors whose essence he holds and they swarm over the cliff to chase after Jack, the same warriors appear three times (in other words, the crew repeats the same frames at least thrice). Look for the green mantis-like creature on the left to verify this. It's not really a goof, just a common trick in animation (especially with background loops), but a little annoying nonetheless.
Phil LaMarr's name is mispelled Pill in the end credits.
Not a goof with-in the episode itself, but for some strange reason, this episode is known as "Jack versus The Five" even though there are only four hunters he has to contend with. Someone can't count.
When Jack enters the village, there's a man turning three pieces of meat on a pole. However, there's no fire underneath the meat.
At first when we see the smell, it comes from a wave behind the mountain. But when Jack is with the dragon it's inside the mountain and it comes from there.
Despite the fact that he calls himself a scissorsmith, the merchant who reluctantly aids Jack apparently does not sell scissors (he appears to run a general store).
The position of Jack's sandals change after he loses one to the stomach birds - first the remaining one jumps from his left foot to his right, then after he is sucked through a valve he has both again. (Some thought if you look hard you can see Jack recover the sandal.)
The dragon's insides appear to be considerably larger than its outside - must be magic.
Why didn't Jack use his "jump good" ability to help him reach the top?
The weapons/abilities for each of the Ultra-robots are as follows:
#1 Circular Saws
#3 Exploding Cords
#4 Grappling claw-hand
#5 Machine guns
#6 Shurikens (throwing stars)
The Scotsman's scabbard changes direction on his back several times throughout the episode as evidenced by which way the hilt is pointing and which hand he uses to pull out his sword.
In the two fights that Jack heard before he was taken into the arena, Gordo didn't bother gloating; so why did he bother to gloat when he saw Jack?
Jack uses the shield to sever the blade-end of the Aqualizer's staff-weapon. However, in the next shot, the Aqualizer holds up his weapon and the blade on the severed end is just visible on it.
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.
So what happened to the time portal? in Aku's temporary shock, Jack could have jumped in it.
Exactly how did Monkey Boy learn to speak English? (Seeing as he confused the words "yes" and "no," he must have taken a crash course.)