Vaporeon: The player loses against the rival at Professor Oak's lab
Flareon: The player wins against the rival at Professor Oak's lab, but loses or skips the battle on Route 22
Jolteon: The player wins against the rival at both locations
In the fourth through seventh battles, Blue has a Potion for each of his Pokémon that he uses 12.5% of the time if the HP falls below 20%.
In the Champion battle, he instead uses Full Restores with the same logic.
This battle can be skipped by obtaining a Pokémon with Cut from a trade, and then leaving Cerulean City by cutting the tree blocking the south exit.
If this battle is skipped, it is possible to break the gameplay sequence by having this battle at any point later in the game (even after other battles against Blue).