T. K. Arispe

 Throwback to Gen I, where due to some questionable battle mechanics, the Psychic type was essentially nearly invincible. Psychic types were supposed to be weak to Ghost and Bug Pokémon, but due to a programming error, the only damage-dealing Ghost-type move in the game (Lick) actually did no damage against Psychic-types. The three damage-dealing Bug-type moves in existence - Leech Life*, Twin Needle and Pin Missile - did extremely low base damage and were basically useless by the time you reached Saffron City. To top it all off, the only Ghost-type Pokémon in Gen I (the Gastly evolutionary line) and most of the Bug-types who actually learned Bug-type moves were also part Poison-type, making them extremely vulnerable to Psychic-types.

I have to give props to Game Freak for taking strides to address this imbalance in Gen II, where they introduced two new types that were both advantageous against Psychic, expanded the move pool for Ghost and Bug moves, and introduced Ghost and Bug Pokémon who weren't part Poison.

That said, considering the astounding complexity of the Pokémon Red/Blue battling system for its time, it's inevitable that the developers would make a few slip-ups, so I'm not really complaining about it. It's certainly not something that breaks the game. You just kinda have to go into it acknowledging that you should probably include a Psychic-type on your team.

*For the longest time, for some reason I thought the name of this move referred to actual leeches, like "yeah man, I'm livin' the leech life stealin' HP from my opponents". It took years before I realized that the "leech" part is supposed to be a verb--the Pokémon is literally leeching life from its opponent. I chalk that one up to being a weird kid who knew way too much about stuff like leeches and hagfish and early Cambrian stem arthropods.