Audio problem: When they are trying to escape Koopa and the little caveman speaks, then Luigi answers him, he has the Princess' voice.

Revealing mistake: After Alley Koop orders his army to attack, he is briefly layered over a Koopa Troopa. (00:04:01)

Revealing mistake: When Luigi trips over a bone, the bone is layered over him. (00:03:30)
Audio problem: When King Koopa throws the snake at the Mario Bros., they use Luigi's voice when King Koopa says "Quick, quick! Run away from him!"