One thing many Pokémon fans have in common is that growing up, we'd hear all sorts of stories about glitches and legendaries you could capture if you did something random and esoteric. Most kids, as we now know, were lying. Or well: it's more like they didn't know how to pull the glitches off, exactly.
Turns out, Pokémon Red, Blue and Yellow have a couple of absurd glitches which allow you not only to level to 100 before the first gym, but also allow you to capture the legendary Mew before the second gym (and then, yes, level that to 100).
I'm not pulling your leg! The hardcore may know these glitches as "The EXP underflow glitch" and "The Mew glitch," respectively. And while these glitches have been documented before, Redditor I_Kissed_Cereal took the time recently to write up tutorials which you can follow to pull off both glitches—and they're complete with pictures.
The EXP Underflow Glitch
First, an explanation—here's Bulbapedia, breaking down why this glitch exists:
In Generation I and Generation II, level 1 Pokémon in the Medium Slow group were calculated to have -54 experience points. However, due to the use of unsigned integers, the game interpreted this value as 16,777,162 experience points. If a level 1 Pokémon with negative experience points completed a battle without gaining enough experience points to reach 0 or higher, the game, attempting to determine its level based on the number of experience points it had, would consider it to be at level 100 (having gone well over the amount required to reach this level), causing it to instantaneously jump to this level.
Which is to say, the glitch doesn't work on all Pokémon. Another curious thing about the glitch is that if the Pokémon has two evolutions, you won't actually be able to get to the last evolution—'mon only evolve once per level-up sequence. So if you try this glitch with a Bulbasaur, it'll turn into a level 100 Ivysaur, not a level 100 Venusaur. But, hey—that's the trade-off you get for fully leveling a Pokemon, a glitch you can pull off before you even reach the first gym.
Anyway, here's the tutorial—click on each image on the bottom of the gallery, from left to right, to see each step:
Or, if you'd like to see the glitch in action, here's some footage by TTEchidna:
The Mew Glitch
This glitch feels like the more unbelievable of the two—but it's real. You can in fact catch Mew early—and then level it to 100. You can't do this glitch as early as the EXP underflow glitch, but still—if you pull it off, you can have a level 100 Mew. All you need is the move growl, lots of Pokeballs, and some patience—just because this glitch is possible doesn't mean it's easy!
Here's the tutorial by I_Kissed_Cereal—once again, click on each image at the bottom of the gallery, from left to right, to see each individual step.
And if you'd like to see the glitch in action, here's footage from MackLL:
Neat, right? Weird to think this was all possible when we first played the games, and that that knowledge did percolate across many playgrounds, but many of us didn't learn how to actually do it until adulthood. So it goes!
It's worth noting that I_Kissed_Cereal is currently writing a whole bunch of different tutorials to glitches you might not know about in early Pokémon games—if you'd like to see them all, check this out. And if you'd like to read about all the documented glitches for all the games, you should read up on Bulbapedia.