Vaporeon: The player loses against the rival at Professor Oak's lab, whether or not they defeated him at Route 22.
Flareon: The player wins against the rival at Professor Oak's lab, but loses or skips the Route 22 battle.
Jolteon: The player wins against the rival at both Professor Oak's lab and Route 22.
In the fourth through seventh battles, Blue has one Potion per Pokémon that he uses 12.5% of the time if their 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).