It lets you get a Lv 100 Mew very early
While you're doing the early Mew glitch, you need to growl at the male swimmer in Mysty's gym's second Pokemon six times. It lowers his attack stat, which is the piece of data the game draws upon to determine the level of the Mew you find. If you do the growl six times, the Mew you find is at Lv 1, which usually isn't possible in the Gen 1 games.
Because of this, it's possible to trigger an underflow error when it gains experience that will max out your experience points. The reason for this is that technically, its total experience is a negative number at Lv 1. Thus, if you gain experience, but not enough to reach level 2, the experience calculator will see that you have this out-of-bounds level of experience, which is an error. The standard response to an out of bounds level of experience is to just reset the number to the maximum legal amount (Lv 100)
Basically, you need to get into a battle and let the Lv 1 Mew gain a little experience but not enough to level up. Then it becomes a Lv 100 Mew.
I didn't do this, because a Lv 100 Mew just breaks the game too much. I want the game to be fun and one-shotting everything gets old quick.
EDIT: To be clear, I still caught Mew, but I just caught him at his "default" level of 7.
Glitches you can take advantage of:
- Early Mew glitch
- Catch Safari Zone Pokemon outside the Safari Zone
- Missingno. (infinite items, glitched Pokemon)
- Glitch City (sequence breaking)
(Warning: the bolded can corrupt your save data right proper)
Other glitches:
- Critical hits are calculated off your speed. This means that faster 'mons should always be given high-crit-ratio moves
- Focus Energy is supposed to raise your Critical Hit ratio. It halves it instead. Dire Hit may work the same way.
- The status effect Frozen is meant to wear off after a number of turns. It doesn't actually wear off unless the frozen 'mon is hit with a fire type move or a healing item
- Great Balls have the same catch ratio at 50% health as at 1 HP (just Great Balls, no other balls are affected by this glitch)
- The super effective / not very effective messages you see during battle might be lying to you if the Pokemon being hit is a dual type. If you hit (say) a Grass/Flying type with an Electric attack, the "It's super effective" message would come up. This is a bug. There should be no message. Even though Flying is weak to Electric, Grass resists Electric, so the damage modifiers cancel one another out. What the game does is to calculate the damage correctly by stacking the modifiers, but lets the second modifier message overwrite the first. (i.e. what should happen: Electric hits Grass "It's not very effective", Electric hits Flying "It's super effective" = No message, what actually happens: Electric hits Grass "It's not very effective", Electric hits Flying "It's super effective" = "It's super effective")
- You can fish in statues. No really. Try it!