The game: Dragon Quest VIII
The background: In a battle against a group of monsters, a player who wishes to attack has to first choose the "attack" option from within the game's "fight" menu, and then pick whichever monster is the player's intended target.
The problem: Once the player has defeated every monster except for the last one, the game no longer waits for the player to pick a target. The reason that's a problem is that the player, having become accustomed to performing two actions for each attack, is now required to adapt to a different mode of operation where each attack requires only a single action. A player who presses twice on the action button while expecting a single attack will be surprised to find out that he's actually chosen to attack twice (once for each player character).
The solution: Have a consistent interface that requires the player to always pick a target for his attacks. Avoiding inconsistencies is more important than saving the player a single button press. Consistency favors the natural tendency for routine behaviors to become automatic, and avoids player confusion.