Stable unstable MissingNo. is a specific iteration of a glitch battle in English Pokémon Yellow, describing how usually regular Yellow MissingNo. will never freeze the game if encountered on real hardware from a save file wiped with Up+Select+B. The save file must also have not encountered other glitch Pokémon with problematic sprites (such as some which cause Hall of Fame corruption), or had the SRAM tampered with in other ways.
This type of glitch battle was documented by the Pokémon Speedruns community, and it is useful for glitches that require unterminated name glitch Pokémon, such as oobLG (within Generation I) and 0x1500 control code arbitrary code execution for Pokémon traded into Generation II. It is also useful for item duplication and its sub-glitches, such as dry underflow glitch.
The name of this glitch is an oxymoron. Fossil/Ghost MissingNo. (IDs 182-184) are often called "stable MissingNo.", yet unstable MissingNo. (that can be invoked using other IDs), while with a high chance of freezing the game, do not necessarily freeze it.
Specifics of the glitch battle and activation
If you clear your save data with Up+Select+B, then an instant encounter regular Yellow Missingno. will set the music bank to 00 but should never freeze the game.
An instant encounter with the hex:32 (dec:50) regular Yellow Missingno. can happen if you escaped from the long range Trainer in Viridian Forest, and then returned immediately after defeating or losing against Misty's Starmie (with a Special stat of 50) by blacking out.
Regular Yellow Missingno. with a save file filled with FF has a special effect.
If a Poké Ball of any sort is used against the Missingno. (as well as glitch items hex:82, hex:83), then the Missingno. will be immediately captured without the rolling animation; making it appear like the ball broke.
The Missingno. will turn into a Rhydon if Cubone is not registered as 'seen' in the Pokédex. Glitch Pokémon turning into Rhydon is expected behaviour for any glitch Pokémon with a Pokédex entry of 000 or inclusively between 152-255 that brings up a Pokédex entry upon capture.
After the battle ends, if Missingno. turned into Rhydon, then the game may ask if you want to give a nickname to "MASTER BALL" (an 11 letter long name longer than the expected limit). If Missingno did not become Rhydon, then the nickname may be "PP UP".
If you choose not to nickname the Pokémon, you secure getting the original nickname; but the nickname field works in special ways for MASTER BALL and PP Up.
If you choose "No" to nicknaming "MASTER BALL" or "PP UP", it will be given that nickname anyway. If you choose "Yes" to nicknaming "MASTER BALL", the nickname field will appear blank initially and if you choose to immediately end nicknaming the Pokémon, it will be named "RHYDON" properly terminated by five 50 bytes. The nickname field automatically fills in with "MASTER BALL" (with the second "L" going off the screen) if you press A on a character. If you choose to end the nickname after this, or empty the name field, you will receive a Rhydon with the same name ("MASTER BALL") as the nickname if you chose not to nickname it. If you don't remove all of the characters, then you can receive a Rhydon with the nickname you choose and it is properly terminated. PP Up:
The behaviour on the nickname screen works the same as "Master Ball", except that if you press A to print "PP UP" on the nickname field and then end the name, its internal name will be "PP UPLL"", rather than "PP UpPOKé " if you chose "No" or blanked the nickname after having it print on the nickname field.