Omi taking an instant liking to the Po Chi game in the pilot makes a lot of sense. A little friend who follows all of Omi's instructions? After failing to get his three new "students" to follow him earlier in the episode, it's no wonder Omi took to Po Chi so quickly.
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.
In the episode The Apprentice, the illustration in the scroll makes the Woozy Shooter look like a rifle or other long gun turned backwards—the purple gas it emits comes out of the part of the Wu that looks like the butt of the gun, and the illustration has it turned backwards so this part is facing towards the user. This may have been completely intentional on Dashi's part; for most Shen Gong Wu, all you have to do is say the name and point it in the direction of your enemies. However, you have to blow into the Woozy Shooter to get it to work properly; someone who doesn't know the first thing about it would probably hold it so that it was facing towards themself, effectively catching themselves in the mist and turning the Wu into a booby trap.
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 FaceHeel 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 FaceHeel Turn in Season 3, he's still wearing his Wudai armor in the showdown, because he hasn't truly changed sides.
Notably the rule about the monks changing into their monk attire from their street clothes was less consistent in early episodes when the monks were still forming their dynamic. Once they were fully set to one, the uniform rule was in place.
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 HeelFace 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.
On a related note, the Sapphire Dragon's weakness to soot seems silly... until you realize that in gem lore, sapphires symbolize water. Meaning that, assuming soot symbolizes fire, what at first seems like a Weaksauce Weakness, turns into a very symbolic version of Elemental RockPaperScissors.
The final not only showed why Rai got to be leader, but why none of the others were meant to be leader. Omi failed to see the consequences of his actions and created two bad timelines of it, Clay was far too passive and wasn't proactive, and Kimiko having been the leader in the first bad timeline, though how much of that lay on that and how much lay on Omi vanishing is up for debate.
Master Monk Guan gave up Dojo to Chase in return for the Spear of Guan, a weapon that he clearly prized. However, in "The Return of Master Monk Guan", we discover that Guan has SEVERAL copies of the Spear made. This would imply that he sold his principles out for nothing ...unless we consider the possibility that Guan had all of those spears made AFTER his victory over Chase. This makes much more sense, as he likely wanted to have several backups made in case another one got lost, and he very likely gave the ''original' spear to Omi.
A few fridges on the two part finale.
The reason Evil Guan is much worse than Evil Chase is fairly simple: the former is a lot more social than the later. Chase pre potion wasn't exactly a social butterfly even when he was friendly, and as a villain he was content sticking to himself. Evil Guan worked with the other villains, making them all more effective.
Why didn't Omi try to use the Ying/Yang Yo Yo's to get Chase to turn good? Several possible reasons beyond Omi/the writers did not think of it.
As a ton of Wu were just stolen, that might have included both of them.
As someone who was turned by the same wu, Omi may not be up for doing the same to even his enemies.
Trying to use them on Chase Young might have been too dangerous: he'd have to sneak up on Chase twice to pull it off, a tall order that could very well just get Omi himself trapped in the Ying-Yang world and risk him being made evil again. Omi would not want the world splitting in half again. However time travel could not be blocked as easily.
Chase probably can put himself into a trance like Master Fung to keep himself from turning, making the task much less effective.
Even if it is a bit really to suggest Chase and Guan knew Dashi personally, Chase knowing Dashi personally could explain his general dislike for many Shen Gon Wu. He probably found things like 'sandals that make your legs stretch' and 'giant golden baby monster that shoots diapers' to be dumb even before he became evil, and kept that opinion when he did.
Knowing about the Shen Gon Wu probably is why he knew what couple to look into before Wuya. He knew to keep an eye out for, say, the Fountain of Hui.
Being Dashi's contemporary was how Chase knew of Sabini despite being stuck in the Mosaic Scale for ages.
Chase looks a fair bit different from Dashi and Guan, and one can not a difference in other aspects. His lair has a almost Roman-esc aesthetic, and Chase is not a Chinese name. As Dashi was a world traveler, it is likely he took Chase along with him from farther afield, perhaps even the Roman Empire (which, 1500 years ago, would either have just broken up or is breaking up).
Why did Omi screw up time travel so badly? Because he has very limited experiences with time travel, and the two times he had done so before were pretty straight forward. The idea of time paradoxes and things going that wrong were something Omi had no context for. Any of his friends have probably seen enough media to know that screwing with time like that is extremely dangerous, but Omi probably has not seen any series that really explains what happens if you try to change history (as opposed to his previous bouts of time travel that did not involve altering history).
Why does the temple mention having potatoes much earlier than they should have? Goof probably, but if it was meant to be an intelligent observation it was probably a reference to the globetrotting Dashi and Dojo bringing some back from South America after hiding a wu or two there.
When Master Fung challenged his students in The Crystal Glasses to try and get the Crystal Glasses off of him, no one was able to. The Glasses lets the user see into the future, Fung said only the wisest would be allowed to use them, only Raimundo refused to actually try, seeing how everyone failed to do so, however, it is entirely possible that Raimundo saw it as a pointless effort if Fung knew what was gonna happen if he was using the glasses, pretty Wise decision mind you.
Raimundo becoming the leader was foreshadowed way back in season 1. He was the first monk to call a "shen yi bu dare" gaining nearly every shen gong wu at the time and making up for his earlier mistake of losing the sword of the storm wu due to neglecting to pay attention. The season 1 finale shows he was very reluctant to leave the elder temple monks to fight off Mala Mala Jong showing his loyalty and returning out of concern. When Master Fung deemed him not yet ready for promotion to apprentice level, he was offended because despite making the situation worse, he felt his loyalty meant nothing and ended up joining the Heylin side by restoring Wuya effectively making a bigger mistake and learning from that as well by resealing Wuya. Along with the Crystal Glasses example above, Raimundo came the longest way making him the most fit to be leader.
As Chase Young had shown a dislike of having people unwillingly join/turn to evil, why would he use the Ying/Yang Yo-Yo plot to reverse Omi's morality. Because, as Wuya and Chase noted in the second part, it didn't really make Omi evil. Omi's tendency towards being a Blood Knight were amped up to a near uncontrollable degree and he's even more reckless than usual (his quick loyalty swearing to Chase that led to Omi-cat in part 3). If anything what the reversing of Omi's chi did was make him take his disagreements with his fellow monks a lot more seriously and personally than usual. Remember Omi didn't join Chase because 'he's evil now'. He did it because Chase 'trusted and believed in him while the Monks disregarded his opinions'. Had Omi not gone into the Ying-Yang World those views would still be there, but they'd be tempered with the hundreds of good interactions and, at worst, been a minor frustration.
Why did Clay get a lot less focus over time? Because Clay, by himself, doesn't do the things that cause many episode plots. Unlike Omi, Rai, and Kimiko he usually doesn't release world ending evils on his own or do crazy things on his own. Most of his focus episodes are caused by others acting on him, usually his family. Like a stone, he doesn't move unless acted on. This is both a good and a bad thing as it makes Clay an excellent student who doesn't cause a thousand years of darkness every other week like the others, but he also doesn't cause plots and he, arguably, doesn't establish himself as a leader.
In The Last Temptation of Raimondo, why did it take Dojo flying out to verify Rai's story? Simple: only two monks have tech to read articles about this sort of thing and one (Rai) has a evil claw on his hand at the moment that probably ruins his typing while the other (Kimiko) is clearly not in the mood to help Rai because she believes he is lying. This is very nice of Dojo, and one might wonder why he did so? Well note that Master Fung is pretty understanding and calm about Raimundo right before Dojo shows up. It's rather possible that Master Fung had guessed or figured out that there was more to this than Rai lying (perhaps he smelled smoke on Rai, perhaps he knows Rai's tells too well, perhaps he suspected that the Heylin were up to something) and asked Dojo to take a look.
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.
In "The Apprentice," Jack visits the monk's temple after his robot duplicate locks him out of his house; Jack says his mother "is baking cookies for him as we speak," which means the robot has essentially taken over Jack's life at this point. This, coupled with the way Jack then tells the monks to "put [him] out of [his] misery" has some...unfortunate implications.
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.
If it went like he planned, there wouldn't have been a problem. Freeze himself and wait for eighty years, get the Sands of Time, then use them to go back eighty years. He'd be back with the gang before they even realized he was gone.
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.