Pomeg glitch

The Pomeg Glitch is a glitch exclusive to Pokémon Emerald that involves the use of the Pomeg. The glitch involves forcing a Pokémon into negative HP by using a Pomeg Berry to lower its Max HP by 2.

Performing the Glitch

Get a Pokemon that has been EV-trained in HP such that, upon using a POMEG Berry on it, its Max HP will decrease by 2. (You can also use 8 HP Ups on a Pokemon at or around Level 60) Now, poison it and walk around until it has only 1HP. Cure the poisoning and use a POMEG Berry on it. Its HP will decrease by 2, but it won't become -1 -- it will instead have ?35 HP (current HP). At this point, the glitch is complete, but there are two very interesting sub-glitches that can be done from this point. SAVE if you want to do both of them.

From this point on, the Pokemon you just glitched shall be referred to as the "POMEG'd Pokemon".

?????????? Subglitch

Head to a PC and make sure that the only Pokemon in your party are the POMEG'd Pokemon and another normal Pokemon. The POMEG'd Pokemon should be at the front of your party. Go to any area and get in a Wild Battle. Defeat (or run from) the Wild Pokemon (best if you beat it in one hit or run from it) using the POMEG'd Pokemon. After the battle, go to the POKEMON menu and switch the POMEG'd Pokemon and the normal Pokemon.

Now, making sure that you don't get in any more battles, head back to a PC and deposit the normal Pokemon. Now return to an area where you can fight Wild Pokemon, but make sure you don't fight ANY just yet. Use any healing item (Potion, Oran Berry, etc.) on the POMEG'd Pokemon and it will faint, but you won't white out and will be able to walk around with no usable Pokemon! Now get in a Wild Battle. Because of the party-switching earlier, and because the POMEG'd Pokemon is now fained, you will now send out ??????????. (which is what happens if you manage to start a battle with no usable Pokemon -- the same thing happens if you have NO Pokemon in your party and enter a battle)

Like most known variations of ??????????, it will have no HP, no attacks, and will faint instantly, so if you don't want to white out, you should revive your POMEG'd Pokemon. (once you revive it, though, it'll turn back to normal)

POMEG EGG Subglitch

Head to a PC and make sure that the only Pokemon in your party are the POMEG'd Pokemon and the EGG you want to battle with. The POMEG'd Pokemon should be at the front of your party. Go to any area and get in a Wild Battle. Defeat (or run from) the Wild Pokemon (best if you beat it in one hit or run from it) using the POMEG'd Pokemon. After the battle, go to the POKEMON menu and switch the POMEG'd Pokemon and the EGG.

Now use any healing item (Potion, Oran Berry, etc.) on the POMEG'd Pokemon and it will faint, but you won't white out and will be able to walk around with no usable Pokemon! Enter a Wild Battle. Because of the party-switching earlier, and because the POMEG'd Pokemon is now fained, you will now send out the Egg!

The Egg is almost exactly like a normal Pokemon -- it has moves, stats, a gender, it's at Level 5, etc. -- except that its color palette is glitched (I suspect it is using the EGG palette). Revive your POMEG'd Pokemon (returning it to normal) and switch out, making sure the EGG doesn't faint. Upon winning the battle, you'll find that the EGG can gain EXP, learn moves, and even evolve!

Of course, by reviving your POMEG'd Pokemon, you return it to normal, meaning that to battle with the EGG again you must perform the glitch again. However, you have to revive the POMEG'd Pokemon or you will white out, even if you win the battle with the EGG. The best way to train the EGG is to mix records with someone who has 6 Lv.100 LATIOS that only know MEMENTO. Then, use the EGG to battle them at their Secret Base; the LATIOS will use MEMENTO and faint themselves, skyrocketing your EGG's EXP.

However, the EGG will always hatch at Lv.5 with no EVs, regardless of how much you train it; however, learned moves and evolutions will be kept. You can hatch a Lv.5 Charizard, a Lv.5 Metagross, or any other Pokemon that you can get by leveling up!

Explanations

POMEG Glitch

The reason your Pokemon goes into ?35 HP is because by lowering your Pokemon's max HP by 2 while its current HP was 1, you subtracted 2 from 1. Your Pokemon's HP should've become -1, but the type of variable the game uses for HP can't store negative values, so if it goes below zero, it "loops" back up to its maximum value: 65535. (it displays as ?35 because the game tries to display 655 as one digit; unlike previous games, however, it just shows up as a question mark instead of a glitched text character.)

?????????? and Egg Subglitch

For the same reason, using a healing item on the POMEG'd Pokemon faints it, because the variable can't go higher than 65535 so it "loops" back around to 0. However, you don't white out because the game doesn't check for usable Pokemon after healing, as under normal circumstances healing wouldn't faint a Pokemon.

Egg Subglitch

An EGG is exactly like a normal Pokemon. The only things different about it are a few values that tell the game it's an EGG. The game sees these values and prevents you from looking at an EGG's true stats or sending it out into battle. The reason you're able to battle with an EGG is because by switching your party around and fainting your POMEG'd Pokemon, you are able to trick the game into overlooking the EGG-specific values.