Why was Razor Leaf "not very effective" against Tentacool in Red?

Question

I'm right now doing Pokemon red, and my venusaur used razor leaf to opponent tentacool.
Obviously, it did critical hit, but something strange is, it says "it's not very effective"
I've hit few more tentacools now and it always says it! Tentacool's water/poison, and it is obvious that grass beats water since it had been always like that since the beginning. What's happening?

Comments

because tentacool is poison

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It's also water, meaning grass type moves would hit it with neutral damage.

Answer 1

This happened to me on a Yellow run recently and I decided to find the actual answer. Essentially it is a glitch, but it's not specific to one person's game. There's a glitch that causes the "not very effective" text and sometimes other text to display even though the actual damage done is unaffected. The attack still does neutral damage due to the Water/Poison typing, it's just that the subsequent text is incorrect.

You can read more about it here at Bulbapedia: https://bulbapedia.bulbagarden.net/wiki/Dual-type_damage_misinformation

Answer 2

Glitched Game.
Tentacool is hurt neutrally by Grass Type Moves.

Comments

Guys, I obviously know that it should be neutral, but it actaully happened?
So is this a glitch or normal thing that gamefreak did not intended to do(like ghost not effecting psychic in gen 1, different from the anime)

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Yes. Exactly like that.

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Can I switch my question a little?
So is this normal thing to happen in gen1(Even it may not make sense), or is it just few people having a glitch?

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Just a few people.

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hmm ok I'll trust you...
Oh wait, my game is glitched? dang...