Mario Club Co., Ltd.
From the Super Mario Wiki
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Mario Club Co., Ltd., credited in most pre-2009 games as the Super Mario Club, is the game testing and debugging team for many of Nintendo's games. Most of the time the club works with Mario games, but they have also worked with other intellectual properties of Nintendo's, like The Legend of Zelda and Pokémon. When credited, they are usually listed under the heading "Special Thanks", but some more recent games list them under either "Debug" or "Testing."
Mario franchise credits