In most video games, poison damage ignores player defenses, making it effective against players who focus on defense. Maple, realizing this early on, fortifies her resistance to it by allowing herself to be poisoned repeatedly. The moment she acquires total immunity to the one thing that can damage her was the start of her becoming an nigh-indestructible player.
Maple putting the [Devour] skill on her shield effectively gave her a counter-attack skill even before she gets the actual [Counter] skill as the mere act of blocking with her shield is enough to deal damage. Also, the skill calculates its damage in relation to Maple's level, so players and monsters whose levels are below Maple's get instantly killed.
Since skills are activated verbally, any player can effectively "invent" a skill by calling out an attack and acting as if one is performing it. Like Sally did. Though subtle, the difference between actual in-game skills is when they are used, there is an echo that can be heard. Frederica not being able to pick up on this was the reason Sally was able to sell her fake skills well.
