Looks like a previously unseen variant of the already-known Chuck-Eat glitch, where item-swapping a Chuck onto Yoshi's tongue will change the reserve item and/or Mario's powerup status depending on the type of Chuck, Mario's previous powerup status, and previous reserve item. In this case the combination of 3 factors happened to lead to invincible Mario + Bullet Bill in reserve.
I have to say I've never seen an enemy in the reserve box before on vanilla SMW, so that was interesting. Not as useful as a cloud or goal sphere though.
I searched more times, and see others videos same trajectory... is not New. btw Sorry.
