A 'flawless' Aerodactyl named CHANSEY, obtained with this glitch. It has a burn but can be healed normally
The Generation I maximum stat glitch refers to a specific sub-glitch of the large storage box byte shift glitch in Pokémon Red, Blue, and Yellow.
It allows for the player to receive a Pokémon with maximum stats (maximum DVs and stat experience), but in one sense was obsoleted by arbitrary code execution in both Generations I and II.
This is a derivative of a glitch discovered by Paco81 and verified by Ciel4, which in one of its most simplest form is called 'remaining HP glitch'. On real hardware, this glitch only works on a box 1 that was never completely filled up. You can get max stat experience and perfect DVs with it, but there are some limitations and downsides of it: the moves you get will most likely be wrong, you apparently have to trade the Pokémon to Pokémon Stadium 2 to be able to get it on Gold/Silver due to it having the wrong types internally and theoretically you may get a freeze if your boxed Pokémon becomes level 1 and its recipient byte is changed to a medium-slow experience group Pokémon. If you're using an emulator track the data before DAB3 and see how the data shifts up and copies an FF for the byte directly before it.
Make sure your Chansey has no FF byte before its move 1 PP address, which has to have 63 PP in move 1 with 3 PP Ups used, i.e. value:FF through the use of the PP underflow glitch. Bytes like this that may be FF through normal gameplay are remaining HP and Trainer ID. If you have an 'unlucky' Trainer ID you may have to use a traded Chansey.
1) A Pokémon with 63 PP on the first move. This is possible with the PP underflow glitch. Get a low levelled Pokémon capable of learning Hyper Beam and Double Team and some healing items. Level 7 Chansey is good for this. It has great HP and low attack. You can get it at level 7 with the Ditto glitch with a Special of 40 or you could trade one from Generation II. Encounter a level 15 Tentacool, make sure Chansey is slower than the Tentacool, use Double Team six times then use Hyper Beam and hit, then hope that Tentacool uses Wrap the next turn and misses. This will cause Chansey to use Hyper Beam again, causing the underflow.
2) A Q, obtainable by evolving 4 4HY, a Yellow exclusive glitch Pokémon at level 6. Ditto glitch Special required: 192. You'll have to trade Q on to Red/Blue if you want to do it there.
3) A box that has never been filled before. This is important. On real hardware DA94, the end marker for a box will always be FF in box 1 if you've had 20 Pokémon. This matters because of the game 'getting' the FF before the one following the series of bytes you want to shift back.
4) Poké Flute, optional. Or you may be able to get poisoned to lower HP.
5) A good amount of Pokémon in box 2, such as 15.
6) Pokédollars. Get over 9998.
Example: Flawless Mewtwo
1) From an empty box 1 deposit the Chansey with move 1 having 63 PP, then 10 Pokémon, then Q in that order. The order is important.
2) Fill your party from another box or through catching Pokémon, switch to box 1 then go into the wild and encounter a Pokémon with a catch rate of 255 and catch it. It will be sent to the first position of box 1.
3) Deposit the Pokémon back in the other box, e.g. box 2, then switch to box 1.
4) Withdraw Q, then the first Pokémon, then the 10 Pokémon, withdrawing the last one every time. You will see no Cancel while you're doing this. To do this, you'll need to deposit Pokémon into another box due to the 6 Pokémon in party limit. When you've done this step, it will max out all IVs, and all 'EVs' except for HP.
5) Withdraw Chansey. Then heal it at a Pokémon Center. Examine the HP modulo 256, e.g. minus 512 if it has 683. This will be the index number of the Pokémon you want. You can reduce the value to whatever you want by playing the Poké Flute and getting hit in battle or theoretically through poison. For Mewtwo, the HP modulo 256 needs to be 131.
6) As you should have no box terminators now, you don't need to deposit the maximum amount of Pokémon you had:
Deposit your glitched Chansey after you've got a remaining HP value you want, followed by any 2 Pokémon and Q, the order is important.
7) Fill your party from another box or catch them, then switch to box 1 and capture a catch rate 255 Pokémon; that will be sent to the first position.
8) Deposit your Pokémon in another box, then switch to box 1 and withdraw Q, then the 1st Pokémon, then the 3rd Pokémon, then the 2nd Pokémon. The last two withdrawals will have shifted Chansey's bytes before DAA8 and written FF, FF to the HP EV. Since current HP byte 2 is located two bytes after the Pokémon 1 'recipient' byte, it gets moved into the Pokémon 1 recipient byte.
9) Take your 'Chansey' into the Daycare and receive it to get your reward, i.e. flawless Mewtwo.
- ChickasaurusGL (both the original author of this article and the video text)