What would happen if Mario took on Pacman in the first level of Super Mario Bros?
In reality, nothing much - Mario exists in a side-on 2-dimensional world, Pacman in a top-down plane. It wouldn't work. They would never know the other existed.
Then again, with a dollop of imagination and a love of roughly-drawn animation, you can make anything work.
Check out this delightful confrontation, courtesy of Imgur: