"Ever wonder what Maga Man would look like on the Atari 2600? Well the guys at the I Am 8-Bit art show did and had a custom Mega Man 2600 game made for their art show on April 17 - May 12, 2007 that could be played on a giant controller. Here is a link to their web page"
What about that says it wasn't running on an actual 2600? There are a lot of people with way too much time on their hands who love doing homebrew programming for ancient systems like this.