The Gen 1 Miss
The bane of speedrunners and frustrating in any playthrough, the Gen 1 Miss exists only in (as the title says) the first generation of Pokemon games and is an unintended effect of the way the game calculates accuracy. Every move in the game with 100% accuracy still has a 1/256 chance of missing, which is why sometimes your attacks will miss for seemingly no reason. The simple explanation is because accuracy was calculated by a value being generated from 0 to 255, where if the number was less than 255, the move hit, and if it were equal to or greater than 255, it would miss. The only unaffected move was Swift, which skips the accuracy check completely.
This is the most frustrating of the “glitches” (though it’s more an unintended consequence than a glitch) because it serves no useful purpose and only annoys when it happens. Speedrunners especially find this frustrating because, on their runs, every single move is planned ahead and requires a high degree of precision. The rest of us just get moderately annoyed and confused…and, you know, throw our handhelds across the room when such a miss keeps us from beating Blue the last time.
Next: Because what we really need is more Magikarp