The slot machine glitch is a glitch in Pokémon Red, Blue, Yellow regarding the slot machines in Celadon City's Game Corner. It was documented by Crystal_.
In the Game Corner, there is a random slot machine that is assigned as the 'lucky slot machine' each time the player enters.
When the player chooses to spin the wheels on a slot machine, there is a 2/256 chance (if the player is using a regular slot machine) or a 5/256 chance (if the player is using a lucky slot machine) that the player spins the wheels in a special "seven and bar" mode (dubbed "mode super") where it is much more easy to win at the slots.
Mode super is apparently affected by a bug; in which it was intended that the wheel advances until a seven appears on the first wheel, or a maximum of four re-rolls is reached, however the actual behavior is that the wheel will advance four times regardless of the symbols on the screen.
Analysis of how the slot machines in Pokémon Red, Blue, Yellow work by Crystal_: