No. If the opponent has increased evasion or you have lowered accuracy, it won't always hit. If that's not what you mean, the answer is still no because it can't steal the opponent's held mail. Also, Pokemon with the Sticky Hold ability will not have their item stolen. Additionally, items cannot be stolen from Pokemon with the Multitype ability. But wait... there are more reasons. :P If the user already has an item or the target is not holding one, it will only inflict damage. Thief will not steal the Griseous Orb if the target or user is Giratina. If the target's consumable held item is triggered by Thief, such as a Colbur Berry or Focus Sash, it will not be stolen.
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