Your rival is the actually the hero, and you're either the real Jerkass or some kind of uncaring unstoppable force. In Lavender Town, he says to you ""Hey, <player>! What brings you here? Your POKéMON don't look dead! I can at least make them faint! Let's go, pal!" before challenging you, without explaining what he's doing there. You then fight him, and if you've been paying attention, you will notice that he no longer has a Raticate. The theory is that the player character wounded their rival's Raticate so severely that it died of its injuries, spurring him on to beat the Elite Four before the player. But shortly after he has defeated the Elite Four, he must fight the player...who defeats him...and then is congratulated by none other than his grandfather, Professor Oak. The former champion promptly gets a scolding and is told he lost because he did not treat his Pokémon with love and trust. This gives the fairly one-dimensional rival character a deep story and portrays him as a tragic hero with terrible luck. It also makes your character's actions fairly despicable, as all this guy wanted was to be a great Pokémon trainer and win his grandfather's love. Granted, this requires some big assumptions (such as his post battle dialoguenote "How's your POKéDEX coming, pal? I just caught a CUBONE! I can't find the grown-up MAROWAK yet! I doubt there are any left! Well I better get going! I've got a lot to accomplish, pal! Smell ya later!") and doesn't change the fact that he acts like a Jerkass towards you.
Professor Oak has had a few theories devised about him over the years. The first type of theory is that he's a Genius Ditz who understands Pokémon but is too stupid to keep all the data about them (or, indeed, remember his own grandson's name), hence why you have to recollect all of it. The second kind of theory puts him in a somewhat more malevolent light, either as having some illegal ties to the various villain teams or as the Bigger Bad of the franchise. Some have also suggested that he is having affair with the player character's mother, and sends a ten-year-old child out into the world on his own to prevent him finding out.
Believe it or not, Team Rocket! The theory posits that all three of Team Rocket's major schemes in the game were actually for the greater good, but they formed an evil organization to accomplish these ends because it would lead to mass panic otherwise. The first time TR is encountered is in Mt. Moon, where they are looking to acquire moonstones and are challenging up and coming trainers to battles (moon stones are later found in the Rocket Base). Later on, a Rocket recruiter is at the end of the Nugget Bridge (in Cerulean City, an important detail) and looks to recruit anyone who defeats the five trainers on it, because they have potential. The third time TR is encountered is in the Rocket Hideout, where you meet Giovanni and see he has a Silph Scope (an item that lets you see and catch ghost-types, which but for a bug are stronger than Psychic types), which you take from him after you defeat him. As well, they were attempting to take the Poké-Flute from Mr. Fuji, which awakens sleeping Pokémon (like the two tanky Snorlaxes found in the game). The final time you encounter the whole organization, they've attacked Silph Co., and are attempting to take the Master Ball (the item that has a 100% catch rate), and are revealed to have been working with many scientists, some of which could have previously worked in the Burned Mansion (where Mewtwo was created). The final time you face Giovanni, he gifts you with Fissure, a 1-hit KO technique. Team Rocket all along were attempting to defeat and subdue Mewtwo. They looked to recruit powerful trainers that could face it, they looked to capture Ghost-types that could trump it, they looked to use the Master Ball to capture and control it, and Giovanni crafted a TM that could defeat it instantly as a last resort.
Americans Hate Tingle: Grimer's and Muk's French names of "Tadmorv" and "Grotadmorv", which are condensed versions of "pile of snot" and "big pile of snot", are unpopular with French Canadians. At least one of the two is guaranteed to come up whenever there's a Dub Name Change debate with Quebecers.note This trope applies because the French translations of the games come from France, and weren't even available in Quebec for the longest time.
Dragonite. It's the original pseudo-legendary, and one of the best Dragon-types in the game (by virtue of being one of only three with the other two being his own pre-evolutions...) Many feel that Dragonite looks like Barney the Dinosaur. The fact that it bears little resemblance to its serpentine pre-evolutions doesn't help matters. Plus, just like Charizard, all the worship it gets from fans who only like the first generation annoys some fans of the newer gens. This may be why they gave Iris's Dragonite a grumpy personality and a generally angry expression in the Black and White anime season, to defy its cuddly appearance and help "unify" its popularity. Incidentally, Red/Blue are the only games in which it is shown with an angry expression, aside from its attack animations in Pokémon X and Y; in fact, every Dragonite card in the TCG from Dragon Vault onwards has shown the Pokémon with a serious expression on its face.
Pikachu, all for being the series' mascot. Either loved for being cute and getting lots of focus, or hated for being overused and promoted everywhere, yet still a very weak Pokémon. Especially the case in Yellow where it's the Starter, walks with the player, and cannot be released or evolved. It's incorporation of elements from the anime such as expressions and Pokémon Speak provided by Ikue Otani, either made Yellow more interesting than the other games, or even more fuel on just how overrated Pikachu was.
The situation is aggravated even more when Raichu is brought up. Where some people feel that its shafted by Nintendo and others feel that its also overrated due to its perceived "underdog" status.
Charizard, despite being one of the most popular Pokémon designs in the series for its dragon-like appearance, is regarded by a segment of fans of the later games as pretty much an unofficial mascot for the widely-despised "Genwunners"—people who prefer the first generation over any other, and are often perceived as doing little but complain about later games. Not to mention, for competitive gamers, Charizard became something of a Tier-Induced Scrappy for several gens. Essentially becoming the Pokémon equivalent of Wolverine in terms of popularity and exposure—including being one of only two Pokémon to get twoMega Evolutions (The other being Mewtwo, who suffers similar problems) doesn't help matters on either end of the spectrum.
As mentioned with Charizard, Mewtwo is very much in the same boat as the original Fire starter—still very popular fandom-wide, but heavily resented by a chunk of fans of the later games for much the same reasons: Utter worship by genwunners and heavy Wolverine Publicity extending to it being the only mon other than Charizard to get two Mega Evolutions. The long-standing, extremely vitirolic Fandom Rivalry between its fans and fans of Lucario and Greninja (who happen to be fan-favorites (forthe mostpart) among those who prefer the later games) in the Super Smash Bros. series and Psychic-types' constant Badass Decay throughout the years does not help things in the slightest. Even so, though, Mewtwo doesn't get quite as much hate as Charizard, though mainly due to having a fair bit less Wolverine Publicity, likely caused by a very negative reaction to the version of Mewtwo featured in the sixteenth anime movie.
Breather Boss: Giovanni is rather easy for being the final Gym Leader, since most of his Pokémon are rather slow and all of them have common weaknesses, or even double weaknesses. Even more so in FireRed and LeafGreen, where his strongest Pokémon in the original Red and Blue—a mighty Rhydon—was replaced by, of all things, its unevolved form, Rhyhorn.
Draco in Leather Pants: Giovanni gets this from some people claiming he wanted to "stop" Mewtwo by using the Silph Scope to obtain a Ghost to fight him and (when that failed) use the Master Ball to capture it. There's no evidence to suggest Giovanni even knew Mewtwo existed outside of the anime (which isn't canon to the games) and the games never explicitly mention why Team Rocket was at Sliph in the first place, so this relies on a lot of Fanon. While Pokémon Origins confirms that they were there for the Master Ball, Giovanni's characterization makes it clear that he's Only in It for the Money, and Mewtwo isn't mentioned until after Giovanni's episode.
Although most of the original 151 Pokémon have become highly regarded in comparison to later additions over time, and a handful having reached superstar levels, at the time of the release of the first generation several Pokémon in particular stood out apart from the starters and Pikachu:
Mewtwo, the strongest Pokémon in the game with the most broken type in Gen I. It is caught at level 70, has a sleek, alien look to it, and outclasses every other Pokémon in the game in pretty much every way.
Alakazam is one of the strongest Pokémon in the game, as it is Psychic, has an excellent moveset and great stats. It is often replaced by Mewtwo, which is better in almost every way, but by the time the player gets that far, Alakazam will have been in the player's party for most of the game especially for those not wanting to include legendaries in their in-game teams. Even when Psychic-types become less powerful andless popular over time as a result, Alakazam still remain strong and fondly remembered.
Gengar is a nasty-looking Ghost-type Pokémon with excellent stats and a cool, creepy moveset. As its evolutionary line is the only set of Ghost-type Pokémon in the game, its immunity to Fighting- and Normal-type moves also helped set it apart. This is particularly noteworthy for the originals, as Ghost-type moves didn't work on Psychic Pokémon owing to a case of Good Bad Bugs. It helps that it gets buffed near-constantly (Powerful STAB move in Gen II, Levitate ability removing its common Ground weakness in Gen III, the Physical-Special split in Gen IV and the nerfs to Dark* Due to the introduction of the Fairy type- and Steel* Steel no longer resists Dark or Ghost-types in Gen VI)
Scyther is a ninja-esque version of a praying mantis, with its scythes being actual metal blades. Thanks to its badass appearance, it quickly became a fan favorite, despite its rarity and lack of good STAB moves prior to Gen IV.
Eevee and its "Eeveelutions" became highly popular for two reasons: first, allowing more than one evolution was a mechanic unique to Eevee at the time, and second, the Eevee-related Pokémon are one and all considered to be adorable.
Gyarados is one of the most physically intimidating Pokémon in the generation, being an enormous, very angry-looking sea dragon. Having come from the incredibly weak Magikarp makes it particularly memorable.
It's probably not a coincidence that all of the above evolutionary lines except the Eeveelutions received Mega Evolutions in Generation VI. The Eeveelutions instead got 5 additional evolutionary branches in future games, for a total of 8 Eeveelutions.
Jigglypuff is a cute pink normal and fairy type that thanks to its popularity has been in every Super Smash Bros game so far.
Missingno. isn't even an actual Pokémon and merely exists due to a programming oversight, but has become an icon of Pokémon history and fanon.
Evil Is Cool: To the dismay of many players, there is no "yes" option when the Rocket grunt at the end of Cerulean Bridge wishes to recruit you into Team Rocket after seeing you battle.
First Installment Wins: These games and the original 150 (+1) Pokémon are pretty much synonymous with the series. Starters from newer games are always compared to Charmander, Bulbasaur, and Squirtle, the fan favourites of Gen 1 typically dominate popularity polls, and Red and Blue/Green Oak are the most well known protagonist and rival respectively. Despite all the glitches and bad balancing, there's little doubt that the originals are the most well-remembered (mostly due to the Pokémania fad). One of the many reasons why Pokémon X and Y is so well-received is because of its huge focus on elements and species from Gen I as well as buffing many fan-favorites from said generation with Mega Evolutions.
Blastoise's French name of "Tortank" is popular with English-speaking fans, having earned special mention on a number of blogs and a Dorkly popularity poll. Since it's derived from "tortue" (which is similar to English "tortoise") and "tank", it can easily pass as a cooler English Pokémon name.note However, the actual French pronunciation is more like "toghr-TONK", not "TOR-tank" (admit it, you probably just read it as the latter).
In an example of American Kirby Is Hardcore, Charizard is prehaps the most popular Pokémon in America, where it beats Pikachu in popularity polls by a huge margin. Charizard merchandise regularly sells out and sells high in English-speaking stores, much more than the actual Series Mascot.
Goddamned Bats: One thing that most have in common is that they appear in places (caves, open water) where you cannot avoid encounters by simply staying out of tall grass as they can appear at any time.
Zubat: They, along with their evolved form Golbat, appear in every cave in the game (you pass through no fewer than four different caves to complete the game). They're not particularly powerful on their own, but their encounter rate is annoyingly high, they're fast (which makes fleeing from them difficult), and they will gleefully confuse your Pokémon with Supersonic at lower levels and the more-accurate Confuse Ray at higher ones. Come the remakes, they're even given the Ghost-type move "Astonish" at low levels and Bite, which they have at higher levels, is reclassified as a Dark-type move, potentially hurting Psychic-types (which are strong vs. their Poison-typing) if you're trying to use them as a counter.
Tentacool: They're basically the Zubats of the sea. You'll run into them while Surfing everywhere. They have Supersonic to confuse you just like Zubat, and add in the ability to poison your Mons as well with various Poison-type attacks while trapping them in battle with Wrap. Another issue with them is that they appear at a wide-variety of levels randomly. Running into that level 40 Tentacruel after mostly battle Tentacool with levels in the high teens even becomes a Boss In Mooks Clothing encounter.
The fact that Giovanni, the boss of Team Rocket, is also the Viridian City Gym Leader is all but common knowledge among fans of the series who have never played the game. Strangely enough, once you enter the Gym, and read the plaque on the statue, it says right there it's Giovanni. And, yet, the character who often greets you at the entrances of the Gyms claims he has no idea who the gym leader is!
Your rival beating you to the title of champion and being the Final Boss after you beat Lance is common knowledge in video game circles.
Junk Rare: There are a number of incredibly rare Mons who provide virtually nothing useful other than to fill up your Pokédex. A few particular examples:
There is only one Farfetch'd and one Lickitung available in the game, and both must be traded for with NPCs. Farfetch'd is just as bad as the Spearow you have to trade for it, and lacks an evolution to make it more useful. It exists basically to show off the trading mechanic in-game and to give you a user for the Cut HM which is acquired very close by.
Tangela can only be caught in one place, has only a 10% rate of encounter there, has mediocre stats, and has a pathetic move set.
Memetic Loser: Charizard, particularly with fans who dislike it and its vocal fanbase, is mocked for being fairly ineffectual in competitive play (especially its Stealth Rock weakness) and for not being a Dragon-type despite looking like a dragon (with some people insisting that it's a mere "lizard"). These points of mockery continue into Generation VI, even though Pokémon X and Y gave it two very viable Mega Evolutions, one of which is an actual Fire/Dragon-type.
Moral Event Horizon: The murder of Marowak crossed it for the entire Team Rocket organization. Ironically, this is softened in the originals for Boss Giovanni, whose Heel–Face Turn is less ambiguous in those games.
Signature Scene: One of the most well-known towns in the game, Lavender Town (or, as it can be accurately nicknamed, the Pokemon graveyard).
That One Level: Silph Co. to first-time players. An immense dungeon, not helped by the labyrinthine layout of the various warp pads. The Card Key is needed to fully explore the dungeon, and there's no clues as to its location—you just have to stumble onto it. Even then, figuring out which warp pad behind which locked door leads to the end of the dungeon is trial-and-error. A veteran who remembers the location of both the Card Key as well as the proper warp to take (and who doesn't care about the plentiful experience from the various Mooks around the place) can beat the whole thing in five minutes, but to a newcomer, it's a nightmare.
The Scrappy: Mr. Mime and Jynx are widely disliked by the fandom for their disturbing designs. Fans of the newer gens also tend to criticize Gen I Pokémon with overly simplistic designs such as Diglett, Magnemite, and Grimer (though evolutions introduced in later gens such as Magnezone tend to alleviate this somewhat).
Tough Act to Follow: Alakazam and Gengar are so powerful and iconic throughout the years that many Psychic- and Ghost-types from the future gens tend to be compared to them respectively to the point that very few of them stand out. Machamp too, but only in comparison to pure Fighting-types.
Ugly Cute: As dopey cartoony hippo-like creatures, Slowpoke and Slowbro definitely qualify.
Unintentionally Sympathetic: Sure, Blue may have been a cocky jerk who got what he deserved by losing his title in mere minutes, but having his grandfather come over just to berate him for not caring for his Pokémon? That's cold.
What an Idiot: The grunt in the Rocket Hideout who drops the Lift Key and says, "Oh no! I dropped the LIFT KEY!" Ummm, just pick it up then, you dummy!
The Woobie: Cubone. You would be too if your mother died after you were born, you couldn't look at the moon because you could see her face in it, and you had to wear her skull to hide your face.
According to the Yellow Pokédex entry, Cubone's weeping echoes inside its skull-helmet. Imagine having to deal with that on top of your grief.
YMMVs that apply to Red and Blue
Alternate Character Interpretation: Missingno. is either a unique and interesting Pokémon if handled correctly or a horrifying abomination that will destroy your game (note that it does not actually do this, but it will corrupt Hall of Fame data).
Arc Fatigue: For first-time players, the trinity of Lavender-Celadon-Saffron. A Snorlax blocking your way to Fuschia City? We'll need a PokéFlute for that. But wait, where is it? Mr. Fuji, who's currently in Lavender Tower? But we can't get past that mysterious ghost near the top! Use a Silph Scope? Where is that? The Rocket HQ in Celadon City? Finally. But then the Silph Co. invasion in Saffron City will get triggered, which might become a Marathon Level for the uninitiated. Definitely a long string of Guide Dang Its that would cause some players to lose steam.
Breather Boss: Bruno of the Elite Four can be easily taken out by any decent Psychic type. This was true in later generations as well, but somewhat downplayed because: a) Psychic Pokémon were no longer Game Breakers, so players were less likely to have them in their teams; and b) Bruno and other bosses in general wised-up and started utilising other attacking types to counter their weakness.
Complacent Gaming Syndrome: The combination glitches, Fake Balance, and the small pool of legitimately good Mons (read: fully evolved and didn't have bad stats) led to the competitive scene being dominated by about 10 of them and lacked any sort of playstyle variation like later generations would have.
Creepy Awesome: The infamous Lavender Town theme is absolutely chilling, and yet so cool at the same time.
Demonic Spiders: Anything that had Wrap or Fire Spin and was faster than your Pokémon was this. All the opponent has to do is use Wrap over and over to prevent you from attacking (AI players have infinite power points, you don't). This was especially bad with Tentacool and Tentacruel, who could couple it with Poison Sting, doing more damage and potentially losing you a Pokémon.
Fandom Berserk Button: Do NOT call Red "Ash" on a Pokémon forum. It will not be pretty. Not helped by Nintendo calling the trainer and the rival with the same names from the anime, before they got the names of Red and Green/Blue in Generation II.
A lot of fans mock Animate Inanimate Object designs in the later Pokémon generations, particularly Garbodor and the Klink and Vanillite lines in Gen V, saying things along the lines of "Pokémon should be based on animals/plants/people." These fans completely forget that Gen I had animate Poké Balls, sentient magnets, and living sludge.
Psychic-types in this game straight-up broke the game, hitting hard and fast with durable offenses and defenses, and the types which were supposed to counter them did jack squat. After the physical-special split in the fourth generation, a downplayed but still highly prevalent echo of all of the above ended up applying to the Dragon-type, resulting in Pokémon like Garchomp and Salamence dominating the metagame for two generations straight before dragons' collective Nerf and the rise of fairies in Gen VI. Ironically despite the attempts for balance the Fairy-type ended up even more broken than Gen I Psychics and Dragons for the same reasons stated above.
Mew is the first Mythical Pokémon—Pokémon only available for a limited time in (often one-time-only) real-life events. In Mew's case, this came about because it wasn't originally intended to be in the game at all, and was only slipped into a vacant data slot at the last moment. As such, real life events were the only way to get it (outside of Good Bad Bugs as mentioned below). Today, an extra thirteen exist, and in an era with WiFi and patches that could be used to insert previously nonexistent species, many fans are annoyed at how behind-the-times the gimmick is presented,note in particular, all Mythicals since Mew were hardcoded into the games years before their official reveal, meaning that there's no way they were snuck in like Mew was and not planned out ahead of time (in many ways a frightening precursor to the dreaded practice of "on-disk DLC"), and how the events are still limited-time one-time-only affairs (and some still require you to go to a real life location, which can be unreasonable if one lacks a car/understanding parents).
Later games in the franchise have been criticized for being too easy, but it was actually Yellow that started the trend of making the campaign easier for the player; giving the player access to all three Kanto starters, making moves more available to some pokemon note Charizard for instance could finally learn fly, and adding some Crutch Pokemon early on to make Brock's gym battle more accessible.
Some Pokémon can only evolve when traded to other players. While annoying and tedious then, later trade evolutions would only get worse, from only evolving when traded for a specific Pokémon, to holding a specific item while being traded. While the introduction of the GTS in later games made getting trade evos considerably easier, getting the ones that need to be holding items was, and still is, nearly impossible for several reasons.
X Accuracy in the first games gave every move perfect accuracy. This includes the one-hit KO moves. The real dangers of this come about due to the eighth Gym Leader, Giovanni, giving you the TM for Fissure, which could be taught to a lot of final form Pokémon—like Dugtrio, who could outspeed most Pokémon easily. And, because OHKO moves could be used to defeat higher level opponents, it made defeating the Elite Fournote Barring Lance, who himself could be defeated fairly easily with a good Ice Beam-user, which almost any Water- and/or Ice-type Pokémon could easily be made into with TM13 a piece of cake. The combination of X Accuracy and OHKO moves was so powerful that the mechanics for OHKO moves had to be changed in future generations so that they could never connect against an opponent whose level was higher than the user's level, and X Accuracy was later nerfed to only give a single-stage accuracy boost that doesn't affect OHKO moves.
The Psychic-type was highest on the elemental tier, as Psychic-types only had a weakness to types of attacks that did below-average damage even after being doubled, and there were very few Pokémon with the stats to use moves of these types anyway (and due to a bug, one of those types actually didn't affect Psychic-type at all, rather than do double damage). And also by the fact that in Generation I, the most common type was the Poison-type (which is weak to Psychic-type), with 33 members, just narrowly beating out the ever-common Water type (32 of the Generation I Pokémon). In later games, the Poison-type Pokémon count fell to the back so far that it was tough to remember why Psychic was ever such a powerful type to begin with. The Psychic-type advantage was made worse by the fact that the stat Special governed both Special Attack and Special Defense. This meant that Pokémon with high Special, such as Psychics, were much more useful than Pokémon with low Special, such as Fighting. This was fixed in later generations by separating them.
Anything that can learn Wrap, Fire Spin, and Clamp, especially because almost all of them have decent speed stats. The main reason for this is because said moves are incredibly dangerous, not allowing the opponent to move for a very long time. Probably why the moves are nerfed for the future games.
Genius Bonus: The Magikarp line is a reference to a Chinese legend that states that if a carp can jump over the Dragon Gate (believed to be at the top of various waterfalls), it will be transformed into a dragon as a reward. This holds more ground when Pokémon Snap shows a Magikarp jumping into a waterfall and emerging as a Gyarados.
Genius Programming: No game of this size had ever been squeezed onto the Game Boy's cartridge before. The Good Bad Bugs in the game mostly came from all the shortcuts the programmers had to make to achieve this. note The majority of the more famous glitches stem from shortcuts the programmers took in order to be able to fit the game within the relatively tiny amount of memory available. The Missingno. and Glitch City bugs are the result of the games having no actual memory protection, so the game tries to create a Pokémon encounter or environment from the wrong sets of data with no failsafes in place to catch the error. As a result, it's possible to break the game to ridiculous extremes and still have it keep on trucking through everything.
Hyper Beam does not require a recharge if it successfully KO's a target or destroys a Substitute. Fixed in Stadium.
Selfdestruct and Explosion will not knock the user out if they destroy a Substitute, though their sprite will disappear. Recoil from moves like Double-Edge is also negated when they destroy a Substitute.
If a Pokémon that had just used Hyper Beam is targeted by a Sleep-inducing move before it gets to move again (before it can activate the "has to recharge" message), the Sleep-inducing attack will always hit and even overrides any status the Hyper Beam user may have.
Psychic-types are actually immune to Ghost attacks when they are clearly supposed to be weak to them.
Leech Seed does extra damage if the target is also inflicted with Toxic Poison.
Haze will cure opponents of any status effects and Leech Seed, and resets Toxic Poison to regular Poison on the user.
HP recovery moves will fail if the user's HP value is 255 or 511 below their max.
Status Buff moves are horrifically glitched out. First, the stat that just got changed will be recalculated from its base level and its buff level. Then, if it was your Mon whose stat changed, and you're in a storyline battle, all badge boosts will be reapplied, including the ones that weren't just negated a moment ago. Then, if the Mon who didn't just move is Paralyzed or Burned, its Speed or Attack will be quartered, even if that stat wasn't recalculated and therefore has already been quartered. But if a Paralyzed Mon uses Agility, or a Burned Mon uses Swords Dance, the stat penalty won't be reapplied, even though it did just get negated.
Focus Energy and the Dire Hit item will actually reduce the chances to crit to a quarter of the previous value. Fixed in Stadium.
Substitute does not protect the user from being inflicted by opponent's Standard Status Effects except for Poison (fixed in Stadium).
If a Pokémon behind a Substitute is inflicted with Confusion and hits itself, the Substitute will take the damage.
Status Buffs can roll over to actually lower the stat after they reach a certain point.
If a Pokémon takes Confusion damage or is fully Paralyzed during the invulnerability turn of Dig/Fly, they will remain invulnerable (and be able to attack) until switching out or using Dig/Fly again.
The Mew glitch, specifically the fourth method of it known as the Ditto glitch (or the "fifth's method" glitch, as it's known in Japan), is performed by having a Ditto copy a Pokemon with a desired Special stat, and it enables the player to have any Pokemon that they want. The fact that one can capture a level 1 version of that Pokemon that will instantly jump to level 100 if the Pokemon gains less than 52 Experience points (done by growling at the Ditto until it no longer has an effect, usually 6 times) makes it useful for getting a high-level Pokemon in a small amount of time.
The Old Man Glitch is one of the many ways you can encounter Missingno., and easily the most well known. It's caused by talking to the Old Man in Viridian City to activate the catching tutorial, then immediately using Fly to go to Cinnibar Island and Surf on the east coast of the island. This will trigger an encounter with Missingno., one of several non-glitch Mons, or a glitch trainer depending on the player's name, since the water tiles on that coast of Cinnibar read the encounter data incorrectly due to a programming oversight.
Missingno. itself corrupts the Hall of Fame data, screws up battle sprites if a Trainer is using it, and don't even try to go for Yellow's Missingno.... but it also thinks your 6th item's 7th quantity bit is its "seen in the Pokedex" flag, giving you an extra 128 of that item every time you encounter it when you're not already holding that many. That makes it insanely useful for getting large amounts of Rare Candies, Master Balls, Nuggets, PP Ups, and other one-use items that are extremely rare (or are just extremely expensive).
Growing the Beard: Pokémon Yellow actually is a better game than Red and Blue in terms of gameplay, as the majority of bugs, glitches, and imbalances are fixed, and the way is paved for the full beard-growing of the franchise with Gold and Silver.
In the Pewter City Museum, there is a model of the Space Shuttle Columbia, which disintegrated in 2003. While it's still explicitly referred to as such in the Japanese version of the remakes (since the Japanese versions were released before the accident happened), in the English version, it's simply referred to as "Space Shuttle".
As soon as the player finishes his objective there, the S.S. Anne would be last seen leaving Vermilion Harbor. Now, think back to the anime episode that loosely adapted that plot...
A Lass outside of the Rock Tunnel complains that there should be a pink Pokémon with a floral print. Now that Black and White are out... Given the reported number of unused designs for Pokémon, many which got used later, it could easily have been an In-Joke at the time.
One of the little quirks of the Missingno. glitch is that, due to not having a back sprite, he'll appear as the last pokémon loaded into memory. This later became Zorua and Zoroark'ssignature power - they'll appear as the last Pokémon in the team.
Many fans had noticed that Hitmonchan's sprite in the Japanese Red and Green resembles Togekiss watching Doduo taking a dump.
The existence of Mew. At the time of the game's release, not even Nintendo was aware it was programmed into the game as a fully functional (but unobtainable) Mon.
Lance of the Elite Four derived a lot of his difficulty from using Dragon Pokémon, which resisted many common types (including the primary types of all the starters, especially notable since starter-only runs are very common back then) and were so obscure that they were almost never encountered in actual battle. A player had to know the type chart really, really well in order to realize that Ice was their one practical weakness. Anyone who has played almost any Pokémon game from Generation II onward is pretty likely to already know this.
It's hard to remember now but Blue being the Champion and Giovanni being the last Gym Leader were originally spoilers.
Seinfeld Is Unfunny: Genwunner views aside, Red and Blue are feature-barren, plotless, and unbalanced compared to later games in the franchise. That said, they (well, technically Red and Green in Japan) started an international phenomenon.
That One Attack: Wrap, Fire Spin, Bind and Clamp. All of these moves disallow your opponent from moving for the duration of the attack. Sadly, since many Pokémon that learn these moves are fast, your Pokémon are highly unlikely to make a move when faced against these foes. Thankfully, they were nerfed in future generations.
Misty's Starmie is effectively almost guaranteed to be faster and stronger than anything you will have at that point and it does very high damage even for mons that resist Water. In FireRed and LeafGreen, her Starmie has switched BubbleBeam for Water Pulse which confuses you about once every three turns it's used.
Sabrina at Gen 1. Her team of Psychic-type Pokémon is fifteen to eighteen (depending on which version you're playing) levels higher than the last Gym Leader. It doesn't help much that Psychic-types were also extremely overpowered in Gen I due to a glitch making them immune rather than weak to Ghost, contrary to in-game advice and Nintendo's own guides, and a poorly-balanced elemental system in which Psychic's only weakness, Bug, had lackluster Pokémon and moves. To rub salt in the wound, the only Ghost types at the time were also part-Poison, creating a vulnerability to Psychic moves. Thank goodness this has been fixed in the remakes.
Lance’s Dragonite in Yellow. In Red and Blue, he wasn’t significantly problematic, apart from knowing Barrier, which Dragonite has never been able to learn. note Until 2016, when he finally gets to learn Barrier legally.... at least, the event Dragonites that are in themselves a reference to Lance's Dragonite. Not here. Got a water-type that knows an ice move, the dragon type’s only weakness? He knows Thunder. An actual ice type? He knows Fire Blast. A rock or ground Pokémon who knows a good rock-type move, since Dragonite is part-flying and therefore vulernable to rock moves? He knows Blizzard. Get ready for a tough fight.
Unwinnable by Insanity: It's possible strand yourself on Cinnabar Island by bankrupting yourself, discarding all of your Poké Balls, and releasing all of your Pokémon bar one. You need to be able to Fly or Surf to leave the island, which you can't do if you got rid of all Pokémon that can learn the moves, and you can't trade them from another game since the game won't allow it unless you have a minimum of 2 Pokémon on you. None of this is remotely possible to do by accident; you have to be actively trying to make yourself stuck.