Spanish and Italian Red/Blue grass surfing glitch

The grass surfing glitch (https://www.youtube.com/watch?v=2jI9LeehZ8E) is a glitch in Spanish and Italian versions of Pokémon Red and Blue, where surfing in the grass via the Surf down glitch gives unexpected Pokémon if a Trainer was encountered before doing the glitch.

Some of the possible unexpected Pokémon include 'M (hex:00), Missingno. and even Mew, depending on the Trainer's Pokémon.

The Pokémon found during grass or cave surfing (exclusively to the Spanish and Italian versions) are taken from the water encounter table (starting at D8A9 and stored as water rate, 7 or possibly more level and species pairs) but the game uses the grass encounter rate.

When a Trainer is encountered, the game writes information about the Trainer's Pokémon to the water encounter table, and after the battle finishes, the Trainer's Pokémon based level and species data in the table is not overwritten as long as the player doesn't go to a place with any water encounters.

Going to a place with no water encounters normally sets the water encounter rate to 0, but since the grass surfing glitch uses the grass encounter rate, the player is able to encounter Pokémon that they are not supposed to.

Further details about this glitch can be found here, including exactly what Trainer Pokémon data corresponds to each level or Pokémon in the list (for example, type 2 corresponds with wild water Pokémon 3's species), etc. http://forums.glitchcity.info/index.php/topic,7344.msg198976.html

Comparison with the old man trick and notes:

The grass surfing glitch is the opposite to what happens during the old man glitch, where the game loads "grass Pokémon" (written based on the player's name during the old man's demonstration) using the water encounter rate.

It seems like the localization team tried to fix the old man trick in Italian and Spanish versions of Pokémon Red and Blue by making it so that when you are surfing the only Pokémon that appear are from the water encounter table, regardless of whether the coast tile is normally considered as "grass".

This seems to have opened up the grass surfing glitch; a glitch which is (in itself but the effects can be replicated with a different method as shown in the English port video https://www.youtube.com/watch?v=NE2DotmvjAs) not possible in English Pokémon Red and Blue.

When attempting the steps for Spanish/Italian Red/Blue in the English and German versions, I got no encounters in the grass at all. In the French version I got grass encounters. But in the Spanish and Italian versions you get water encounters.

Interestingly, if the player uses a walk through walls glitch to walk on the Cinnabar Coast in Spanish and Italian versions instead of surfing, then they can do the old man trick like in English versions.