Some of the Shen Gong Wu hiding places don't make much sense. Take the Eye of Dashi, for example (hanging from a tree branch), or the Changing Chopsticks (Hanging out of the nostrils of an Easter Island head). Are we supposed to believe those Wu remained where they were, unnoticed and undisturbed, for 1500 years? Perhaps the worst offender is the Mikado Arm, which was on top of a skyscraper in mid-construction (What, did someone just carry it up there on a whim?)
Some of them got found by people that didn't know what they were, like with the Star of Hanabi. Just imagine that every Wu has a story behind how it got there.
While Jack Spicer of Xiaolin Showdown is considered an Ineffectual Sympathetic Villain, you'll notice that he's surprisingly fast and flexible, and can accomplish things like air somersaults even without the Mantis Flip Coin. Well, in one episode, it's revealed that Jack's mother sent him to figure skating lessons, which is why his body is trained to accomplish all the previous athletic feats. In connection with that, Jack is good at challenges that involve snow and/or ice, most prominently seen in Ring of the Nine Dragons where he was snowboarding quite impressively even before he cheated; this is most likely because the lessons took place in snowy or icy areas where Jack would have learned the other sports as well.
Why is it that when Omi used the Ring of the Nine Dragons, even three clones resulted in them being immensely stupid (to the extent that one couldn't even get past level one of a video game) but when Jack uses it (the second time, for about eight clones) they can still perform normal activities and even comprehend rules and orders? Well, despite acting like an idiot in general Jack still isa genius, and so he has a larger IQ to distribute, resulting in each clone getting an average IQ instead of incredibly low ones like Omi's.
Why did Master Dashi seem so unsurprised to see Omi in "Days Past"? Because he already knew Omi from the events of "Time After Time!"
Master Monk Guan's nicknaming Raimundo "Bobo." Rai's from Brazil, where the official language is Portuguese. In Portuguese, "bobo" means "silly."
The Fountain of Hui, the Shen Gong Wu which provides limitless knowledge, is quite possibly named after the show's creator, Christy Hui, who is the person who has the most knowledge of the show's universe.
Alternately, it could be named after 慧 (hui), which is one Chinese translation of "intelligent", or 智慧 (zhi hui), which means wisdom.
Why does Omi get jealous whenever his friends (eg Jermaine, Ping Pong) receive attention from the other monks? Its because they're his friends. Obviously he likes the person in question, so when the others start recognizing them, Omi is afraid that he'll be replaced. He then tries to prove he's better so that he won't be left behind, but because he does not get people, he ends up making everything worse before he lets go of his jealousy and admits to his mistakes.
How is it that, no matter how many times a Monster of the Week steals the entirety of the Vault, the Monks still have Shen Gong Wu for the showdown? Because they are smart to hide Shen Gong Wu all over the temple.
Why are so many Shen Gon Wu similar in powers? Because they are Xiaolin Monks, plural. Dashi had a team just like the current monks, and if they had to go up against a foe weak against lightning, it would be best to have multiple types of lightning wu to increase the odds of victory.
In hindsight, the fact that evil can never truly be defeated should have been obvious. Why? Because Dashi, who at one point had both Shen Gong Wu needed to have the knowledge to permanently destroy evil, was a good guy. If it were possible to destroy evil and he had the means to do so, he would have done it. Since he didn't, then of course that means that it can't be done.
In Xiaolin Showdowns, what a person wears seems to hint at where their loyalties lie. When Raimundo goes through his Face–Heel Turn in Season One, he's wearing his street clothes in his Xiaolin Showdown—because while he hadn't turned on them yet, he was no longer affiliated with the Xiaolin Dragons in spirit. In contrast, when he pretends to do a Face–Heel Turn in Season 3, he's still wearing his Wudai armor in the showdown, because he hasn't truly changed sides.
Omi's treatment of Rai after his betrayal may be due to the fact that Omi still maintained faith in him (in fact, believing Rai simply faked going evil) and took it the hardest upon learning Rai did do a Heel–Face Turn. He might be harboring some negativity for that.
Why does Dojo have problems breathing fire in the episode "Night Of The Sapphire Dragon"? Well, it's because he's in the wrong size. Take a look at the four episodes of Xiaolin Showdown where he uses this ability as an attack. In "Journey Of A Thousand Miles", Dojo grows to his giant form and breathes fire, to show Raimundo that he is not a gecko. In "Enter The Dragon", a 2 headed Dojo, charges up and breathes fire to force Omi out of a tunnel. In "Saving Omi", Dojo breathes fire to protect Kimiko, Raimundo & Clay from Chase Young. In those 3 episodes he is in his giant form, when he breathes fire. Now in "Night Of The Sapphire Dragon", Dojo is in his small size for the entire episode, which is why he is having problems breathing fire without assistance (Clay's hot sauce). In other words, Dojo needs to be giant-sized to breathe fire normally.
One of the ingredients needed for the Lao Mang Long soup is a dragon, which might explain why there aren't many dragons in the XS universe.
The very idea of "1000 Years of Darkness" is very frightening by itself. No sunlight means no plant growth, which can be catastrophic for animals and humans alike. Being ruled by an amoral immortal doesn't help any.
Here's a crazy theory on how Guan is still around after 1,500 years and Dashi isn't: Let's say Xiaolin monks really don't die of old age, like how Master Fung is still around after 80 years in "Time After Time: Part I", but they can be killed by other means, like how Raimundo, Kimiko and Clay were apparently 'killed' by Jack's giant mecha in that same episode. With Dashi, what if after Guan 'went away', Dojo underwent his destructive urge for Shen Gong Wu from "Enter the Dragon", exactly 1,500 years before that episode, and KILLED Dashi in the process?! Keep in mind that he'd hidden all the Shen Gong Wu at this point and would've had no way to defend himself from Dojo's wrath.
In the series finale two-parter, Omi freezes himself for eighty years to get the Sands of Time back. He doesn't tell anyone but Dojo who comes with him. Now think of what everyone else knows... nothing. One minute he's there the next he's gone. His poor friends probably spent eighty years looking for him to no success. Omi is a small child with a LOT of enemies, for all they know he's dead somewhere. This is probably the single most selfish thing Omi has ever done.