From Glitch City Laboratories
An oversight exists within Pokémon Yellow, which allows the player to increase a following Pikachu's happiness via a recovery item without restoring any HP. As a result, players are able to take advantage of any recovery item such as a Potion and use it without limit to quickly raise Pikachu's happiness after finishing a battle.
In addition to players using this glitch so that they can increase Pikachu's happiness value, it is often used at early points in the game so that players are able to adopt a starter Pokémon as soon as they reach Cerulean City, Route 24 or Vermillion City. Since an item such as a Potion; when used, would not decrease in quantity due to the message "It won't have any effect." players might choose to use the Potion a certain number of times before entering a battle to make Pikachu have a maximum happiness value of 255.
When the player uses a recovery item, regardless of whether the "It won't have any effect" message appears or not, the new happiness value (values of +5, +3 or +2 depending on the following Pikachu's current happiness) is stored in an extra byte before being applied to Pikachu when the player enters a battle. If the player is already in battle and uses a Potion, these changes are applied instantly, regardless of whether Pikachu has full HP or not.
Since instant changes to Pikachu's happiness often occur in game, for example when the following Pikachu faints in battle its happiness value decreases even though technically no HP has to be drained in this process (as Pikachu's happiness value does not decrease simply when it loses HP); it is presumable that perhaps the game uses no exception handler to stop the player increasing the following Pikachu's HP via a healing item when its HP is full.
- Mystery Gift shop oversight
-  A video entitled "Have Pikachu instantly like you" which takes advantage of the Pikachu happiness oversight.