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Loophole 1 - Level 0 trade evolution loophole:
Thanks to the Pokémon Speedruns Wiki for this oddity.
When the player receives any valid level 0 Pokémon that normally evolves by trade, it will not evolve.
The cause is that the developers specifically coded that the Pokémon should evolve by trade only from level 1 and up.
In the data, the format for Pokémon evolution is xx (evolution type), yy (level, stone), zz (Pokémon to evolve into). The xx value dictating an in-game trade is 03 (trade).
Machoke for instance has the data 03 01 7E. It will evolve by trade into Machamp only at level 1 and up.
Modifying the 01 to 00 with a hex editor or Game Genie (e.g. Yellow code: 005-888-E6E) will let it evolve at level 0 and up.
Other xx values for reference are 01 (level up) and 02 (evolution stone).
If you try modifying the 00 in the data at the following offsets in Yellow to 01, then you may be able to remove the loophole (though I have not tested all of them).
For further information about the evolutions (and level up moves structure), this may be useful. http://forums.glitchcity.info/index.php/topic,5217.msg135859.html#msg135859
This glitch occurs both when communicating with another player and with an "in-game trade" where the Pokémon you receive will evolve (for in-game trades the Pokémon you receive is the same level as the Pokémon you send).
Cubone for Machoke - Route 5 (Yellow).
Machoke for Haunter - Cerulean City (Japanese Blue)
Kadabra for Graveler - Cinnabar Island (Japanese Blue)
Example method to obtain a level 0 Cubone in Yellow (text pointer manipulation):
Trick from the Pokémon Speedruns Wiki:
This requires that you have never picked up the Max Potion (hex:11) item ball on the third floor of the Pokémon Mansion on Cinnabar Island and requires an Ultra Ball x84 that will be placed in the map's text pointer value (determining the source of text for each ID).
This item ball (text ID 3) will be forced to bring up text from a Trainer text box from Route 24), and since for encounters IDs less than 200 are Pokémon and IDs greater than 199 are Trainers, you will encounter the hex:11 Pokémon - Cubone at level 0.
1) Get an expanded items pack. Prepare Ultra Ball x84 in it. Using the dry underflow glitch, an items pack of 255 is achievable if you can get a x255 item stack.
This can be done by encountering Missingno. and duplicating the sixth item by 128, tossing at least two to get 127 and encountering Missingno. again. Alternatively if the item is usable and is not a Poké Ball, you can get the 129 stack in the first battle, use up two of the item in battle to get 127, keep this stack in the sixth slot and capture Missingno. to get a 255 stack.
Unfortunately Yellow (nor Japanese Blue) have the old man trick to see Missingno. easily, so another method to get a x255 stack must be used such as the Ditto trick (recommended with a stable Missingno; Special 182, 183 or 184) or the Cable Club escape glitch with a stable Missingno. http://pastebin.com/DfwS1Qbn
2) Go to the Cinnabar Mansion's 3F. Put the Ultra Ball x84 into item 40 (which should be "BLUE") and interact with the Max Potion item ball.
3) You will encounter a level 0 Cubone which you can capture. If you have an empty party space, it won't go to the box and won't revert to level 1 after being withdrawed. This level 0 Cubone can be traded for a level 0 Machoke that will not evolve.
Loophole 2 - Items with quantities of 0 not activating events:
I'm not sure if this has been documented before. Various items that should have an event in the game when they are in the inventory, such as the Silph Scope revealing the identity of a ghost will not work when the quantity is 0.
They can be selected; so for example, it is possible to view the number of coins in a Coin Case; but the game would act as if there is no Coin Case when the player tries to play in the slots, or buy more coins, or try to exchange coins for a prize.
Key items have internal quantities, however, the quantity can normally not be seen, so you could trick your friends with a Silph Scope that doesn't work, a fake S.S. Ticket, etc.
Most items with quantity of 0 can be obtained with TheZZAZZGlitch's any item trick. https://www.youtube.com/watch?v=98_azamLeh4
Applicable items (may have missed some):
Bicycle, Old Amber, Dome Fossil, Helix Fossil, Secret Key, Bike Voucher, S.S. Ticket, Fresh Water, Soda Pop, Lemonade, Gold Teeth, Coin Case, Oak's Parcel, Silph Scope, Lift Key.