Mew glitch (Special encounter) with no Trainers at all

Read description for more information:
https://www.youtube.com/watch?v=YRoN7_KWTTU
Steps for Pokémon Red and Blue:
1) Prepare an item x1 in the sixth position of your items pack and a Pokémon with the Special stat relevant to the Pokémon you want, for example, a Special stat of 21 is Mew. For other values, please see the Big List. http://glitchcity.info/biglist.htm.
2) Talk to the old man glitch and watch his catching demonstration by choosing 'no'.
3) Fly away to Cinnabar and surf along the Cinnabar Island coast until you find a Missingno or 'M.
4) When you find one, the quantity of your sixth item will increase by 128, becoming 129.
5) We want 255 of the sixth item. You have two options. You can run from the battle, toss two of the sixth item and encounter Missingno./'M again to get 255. Alternatively, if the item is usable in battle and is not a type of Poké Ball, e.g. X Attack and Potion works, you can use up two of them and catch the Missingno./'M to get 255 with only one battle.

6) Go to Route 15 (east of Fuchsia City) and encounter a Ditto. Have the Ditto transform into a Pokémon with the Special stat relevant to the Pokémon you want.

7) Without encountering another Pokémon, fly to Cerulean City and enter the Pokémon Center.
8) Deposit the item x255 into the PC, and have at least two other tossable items there.
9) Arrange your PC like this: Slot 1: Item x1, Slot 2: Item x1 Slot 3: Item x255.
10) Toss the items in slot 2 and slot 1 to get three stacks of your item x255.

11) Keep tossing 255 of the item in the first slot until you can only select two items. The number of times you need to toss 255 is equal to the number of items you had originally minus two.
12) Toss 253 of the first item, then swap item 1 with item 2, twice.

13) If everything was done correctly, the quantity of the first item should become 0, and you will be able to scroll past the 50th PC item to access unrelated memory addresses.

IMPORTANT: From now on, do not do anything not listed here with your items past item 50 and save unless you know what you're doing or you may mess things up in a bad way like map event variables.  In particular, only have one of item 100, other values may correspond to unwanted glitch scripts.

14) Scroll to the 100th item (which represents D601 in Red/Blue or D600 in Yellow) and toss 255 of it to get "j." x01. Now, exit your PC, flash the start menu and close it and go north of Cerulean City. A level 7 Pokémon will appear just like if you used the long-range Trainer/Trainer escape glitch.

Steps for Pokémon Yellow:
Pokémon Yellow does not have the old man trick, but if you have a link cable then there is an easy way to duplicate your items with no Trainer involved. As in Pokémon Red and Blue, you will want to prepare a Pokémon with the Special stat relevant to the Pokémon you want. (decimal values here http://glitchcity.info/biglist.htm)

1) In Pokémon Red and Blue, catch a Ditto. Take it into battle and use Transform on a Pokémon with more than one move. Swap the first move with the second (or with the third/fourth if you want) and exit the battle. This Pokémon should now know "-" as the first move (also known as the CoolTrainer move).
2) Trade this Ditto into Pokémon Yellow. In Pokémon Yellow, Fly somewhere close to grass/whatever is needed for a wild encounter.
3) Put an item x1 in the sixth position of the bag and prepare some Ultra Balls.
4) Put the Ditto in the first position of the party and fight a wild Pokémon. You can fight the wild Pokémon anywhere unlike in Pokémon Red and Blue's CoolTrainer glitches.
5) Open the Pokémon menu or the items menu and then close it (very important because you need to update some screen data for this to work, the game will likely freeze otherwise and I unfortunately can't guarantee that the freeze won't be harmful to your save) and then keep opening and closing fight until the music fades out.
6) Throw an Ultra Ball and if you are successful, the game will say that you caught a Missingno. (you do because the enemy Pokémon in battle was changed due to the move). You should now have 129 of the sixth item. I'm unsure what would happen if you are unsuccessful but there seems to be a very high chance of success. But just to be safe it may be a good idea to save before this.
7) Toss two of item six to get 127, and repeat steps 4-6 to get 255.

8) Fly to Cinnabar Island with a Super Repel/Max Repel and an Escape Rope (recommended).
9) Enter the Cinnabar Mansion, use your Repel with a high level Pokémon up front if you want, and go up the first ladder you see. You will want to get to B1F (see steps 10-13 below)
10) Go up the ladder up from the entrance carpet.
11) Go up the northern most ladder (not the closest one), then press the switch.
12) Go down one of the first two holes from the left.
13) Go down the ladder to reach B1F.
14) Keep walking around until you find a Ditto.
15) Have the Ditto transform into the Pokémon with the relevant Special stat.

16) Without encountering another Pokémon, fly to Cerulean City and enter the Pokémon Center.
17) Deposit the item x255 into the PC, and have at least two other tossable items there.
18) Arrange your PC like this: Slot 1: Item x1, Slot 2: Item x1 Slot 3: Item x255.
19) Toss the items in slot 2 and slot 1 to get three stacks of your item x255.

20) Keep tossing 255 of the item in the first slot until you can only select two items. The number of times you need to toss 255 is equal to the number of items you had originally minus two.
21) Toss 253 of the first item, then swap item 1 with item 2, twice.

22) If everything was done correctly, the quantity of the first item should become 0, and you will be able to scroll past the 50th PC item to access unrelated memory addresses.

IMPORTANT: From now on, do not do anything not listed here with your items past item 50 and save unless you know what you're doing or you may mess things up in a bad way like map event variables. In particular, only have one of item 100, other values may correspond to unwanted glitch scripts.

23) Scroll to the 100th item (which represents D600 in Yellow) and toss 255 of it to get "x." x01. Now, exit your PC, flash the start menu and close it and go north of Cerulean City. A level 7 Pokémon will appear just like if you used the long-range Trainer/Trainer escape glitch.

Notes:
After encountering the Pokémon, the quantity of item 100 is reset to 0, but you are free to change the quantity of item 100 to 1 again. If after encountering e.g. a level 7 Mew you immediately go north of Cerulean City again, you will encounter a Pokémon based on the Special stat of the Mew. The Pokémon are not constant due to the Pokémon having random DVs so it may be a nice surprise to see what Pokémon appears to keep activating Special encounters one after the other, however, one possibiltiy (regular Missingno.) will likely freeze the game in Pokémon Yellow.

Technical explanation of this trick and Trainer-Fly:
When you fly away from a Trainer, the game writes 01 to an address past D5F1, and the address depends on the place. For example, D600 (D5FF in Yellow) is for Route 8 (west of Lavender Town) and D601 (D600 in Yellow) is for Route 24 (north of Cerulean City). These addresses activate a script on the route.

A value of 01 means that upon returning to the route the previous text box is loaded based on the text ID in CF13 (often 00 start menu), which can be a completely different text box if the same text ID corresponds to another text box in another map. e.g. Lavender's text box 07 "We heal your Pokémon - Pokémon Center" and 07 on Route 8, a Lass battle "We must look silly standing here like this!" After the text box is closed, you will get an encounter if the value at D059 was not written as 00. After the battle ends, the value is reset.

This script is normally disabled if the value at D730 is 01. When you approach a Trainer, bit 1 is set for this address (which tells the game to make the Trainer keep walking) and if you flew away from them it is never reset, meaning that you cannot get the text box or the encounter. However, bit 1 of D730 is reset after you battle another Trainer who walks up to you, when Professor Oak is done walking after beating the Elite Four, or when you are guided by the Pewter Museum guy in Pewter City. This is why you need to battle another Trainer or follow a route to have the Pokémon appear.

But a better question is why the Special stat determines the Pokémon and the attack stage modifier (normally 7, neutral) determines the level.

Háčky from the Glitch City Laboratories forums very recently explained this.

"When a Trainer spots you, the game loads the Trainer class into $CD2D and team number into $CD2E, and sets a map script that will have the Trainer give his/her dialogue and start the battle. When you leave the map, the current script for that map is saved, and will be run again when you re-enter that map.

In battle, $CD0F–CD1F are used to store the stats and stat modifiers of your active Pokémon, and $CD23–CD33 are used for the enemy Pokémon. This data consists of the Pokémon’s level and unmodified stats (HP, Attack, Defense, Speed, Special), followed by the modifier stages for Attack, Defense, Speed, Special, Accuracy, and Evasion. Thus, $CD2D is the lower byte (big-endian) of the enemy’s Special stat, and $CD2E is its Attack modifier. When you return to the map where you began the glitch, the map script will read these values and start a battle against whatever Pokémon or Trainer they indicate."

Special thanks: http://forums.glitchcity.info/index.php/topic,7256.0.html
http://wiki.pokemonspeedruns.com/index.php/Pok%C3%A9mon_Red/Blue/151_Pok%C3%A9mon#Box_Underflow_T-Fly_Variant