Gen 1, Japanese Releases

Introduction

The Generation 1 mechanics most players know is already very different mechanically from the later generations of Pokemon. However, up until Generation 5, Pokemon Games initially released in Japan before releasing to the rest of the world, usually with glitches patched or changes to moves to balance the game. The first generation of Pokemon has examples of both, though only a handful of changes actually affect battles between players, these differences are as follows.

Blizzard Freezes its target 30.1% of the time.
Swift does not ignore accuracy checks except against substitutes.
Draining moves, such as Absorb or Mega Drain, fail on substitutes. Funnily enough, Draining moves not failing against Substitutes was a glitch introduced when the games were internationally released which eventually became a feature of later generations.
Some Pokemon for access to Japanese event-only moves such as Pikachu with Fly and Fearow with Pay Day.