Old Man Glitch

Prerequisites:

To perform the Old Man Glitch, you need:

The Soul Badge from Koga in Fuchsia City
A Pokémon that knows Surf (HM03)
A Pokémon that knows Fly (HM02)
To have visited the Pokémon center in Cinnabar Island

Demonstration

In Viridian City, there is a particular old man who's passed out and blocks your path before you obtain the Pokédex. After you get the Pokédex, he will have gotten up and will be walking around the area.

If you talk to him, he will ask if you are in a hurry, and if you say no, he will teach you how to catch Pokémon. In the catching demonstration, he will catch a wild Weedle that shows up out of the blue. He will show you how to catch this Weedle as many times as you want if you just talk to him again. Sounds pretty neat, doesn't it?

Despite being so rudimentary, this tutorial is always available, even when you're at the very end of the game. If you've already gotten to Cinnabar Island, then after completing the tutorial, you can immediately fly over to Cinnabar Island, walk to the eastern shore, and surf alongside the shore tiles...

HUH!??

What just happened?

Named after the Old Man in Viridian City, the Old Man Glitch allows players to encounter several rare Pokémon, including glitch Pokémon such as the infamous MissingNo.. During the catching tutorial, the game temporarily replaces the player's name with OLD MAN.

To make sure your original name isn't lost, the game copies your name to the currently loaded grass encounter table. After the tutorial is over, the game restores your original name.

However, the game does NOT clear or initialize the encounter table after the tutorial. The game also does not clear the encounter table if you enter a map that does not have a grass encounter table. Viridian City, Cinnabar Island, and Route 20 do not have a grass encounter table, which makes sense, because neither are supposed to have any grass tiles.

The tile

Bodies of water in Kanto usually have these tiles lined up along their western shores. These tiles can be surfed on, and just like other surfable tiles, they may produce a wild encounter if surfed on.

However, in regular gameplay, you might have noticed that the Pokémon encountered while surfing on those tiles are not the same ones as the ones that spawn in deeper waters; they are the same as the ones that spawn in nearby patches of grass. Due to a programming error, these tiles encounter Pokémon from the currently loaded grass encounter table rather than the water encounter table.

So what was in the encounter table!?

That depends on your chosen name. The player's name consists of 11 characters, but it is only possible to directly set 7 of those with a custom name. In addition to the three characters in the naming screen, there are two special characters. These are the "end of name" character (ID 80), and the "null" character (ID 0).

Characters are 8 bit values, so there are 256 possible characters, but only some of them can appear in your name normally. Pokémon IDs are also 8-bit values, so there are 256 possible Pokémon that can appear. However, several of those are glitch Pokémon, and encountering Pokémon with IDs past 200 will result in a battle with a glitch trainer. Pokémon's levels are also 8-bit values, which means they range between 0 and 255. The game only allows leveling up to level 100, and any Pokémon above that level will go back to level 100 if it levels up.

Suppose our theoretical player character has the name ULTPoke. This name takes up all 7 characters that players are allowed to set, but there are 4 unseen characters as well. The 8th character is the "end-of-name" control character mentioned earlier. It is followed by three "null" characters. All custom names have an "end-of-name" character after their last letter, immediately followed by a series of "null" characters.

The encounter table interprets ULTPoke as a table of 5 Pokémon with their own levels: (the slash (/) characters represent invisible control characters)

U148 L139 T147 P143 o174 k170 e164 /80 /0 /0 /0

Every green character represents a Pokémon that can be encountered, and every red character represents the level of the Pokémon represented by the green character in front of it. The numbers above the characters represent their indices.

This page lists the internal ID numbers for each Pokémon in Generation I, including glitch Pokémon. The reason so many MissingNo. are interspersed between other Pokémon is because those slots used to be occupied by prototype Pokémon that were eventually scrapped. The programmers were too lazy to clean up and rearrange the indices, so they just dummied out the data.

In our case, the encounter table consists of:

	HAUNTER Lv. 139
	MISSINGNO. Lv. 143
	RAPIDASH Lv. 170
	Lv. 80
	Lv. 0

The following table shows all possible name characters and what Pokémon and levels they result in for the Old Man Glitch: ("\0" represents the null character, "\e" represents the end of name marker)

Characters that are available in the naming screen but are not listed here result in encountering a glitch trainer. Unless you know what you're doing, you'll probably want to avoid those. If you want to encounter a glitch trainer on purpose, this page lists the characters and IDs that result in glitch trainers, and this page lists some of the possible glitch teams that you can get using this method, depending on the last trainer you fought.

What about preset names?

When you first start the game, you are given the choice of three preset names. These names are RED, ASH, and JACK if you're playing Pokémon Red, or BLUE, GARY, and JOHN if you're playing Pokémon Blue.

Internally, these names are stored as the following strings: (the slash (/) characters represent the end of name marker)

RED/ASH/JACK/NEW NAME

BLUE/GARY/JOHN/NEW NAME

Players who selected these names get the following encounter tables:


MEWTWO Lv. 132	MISSINGNO. Lv. 146	GOLBAT Lv. 128
GOLDUCK Lv. 80	MISSINGNO. Lv. 80	MISSINGNO. Lv. 138
MISSINGNO. Lv. 146	GOLBAT Lv. 128	SNORLAX Lv. 141
MISSINGNO. Lv. 80	MISSINGNO. Lv. 138	MISSINGNO. Lv. 150
GOLBAT Lv. 128	SNORLAX Lv. 141	GOLDUCK Lv. 141


ABRA Lv. 139	MAROWAK Lv. 128	MISSINGNO. Lv. 142
MISSINGNO. Lv. 132	MISSINGNO. Lv. 152	MISSINGNO. Lv. 141
GOLDUCK Lv. 134	CLEFABLE Lv. 137	SNORLAX Lv. 141
STARMIE Lv. 145	ELECTRODE Lv. 135	MISSINGNO. Lv. 150
MISSINGNO. Lv. 80	ELECTRODE Lv. 80	GOLDUCK Lv. 141

No matter what name you pick, you are guaranteed to have MISSINGNO. or in your encounter table. This makes the Old Man Glitch the ideal way to trigger the next glitch on our menu, Item Duplication!!