From the Super Mario Wiki
Bomb Mario is a form that Mario can only assume in Super Mario Land 2: 6 Golden Coins. The form does not appear in the game itself, but rather during the Select Screen. If Mario, instead of choosing one of the three Warp Pipes to continue a saved progress, jumps beneath the "Clear" block to the right, he will transform into Bomb Mario. When now selecting a Pipe, the saved data will be deleted. Bomb Wario would go on to appear in Wario Land: Super Mario Land 3, which used an identical method of deleting files.
This form appears again in Virtual Boy Wario Land, where Wario once again uses it to delete save data.
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