Mario is one of Nintendo's most popular game franchise. With loads of awards and nominations, it is the greatest video game in game history. To compete with other video games, there have been on-going arguments and debates between Mario and Sonic about which game is better. To top it all off, they'd say that Mario is better.
So they tried to make a video game crossover with Sonic & Mario to help the discussion. A video game titled Mario vs. Sonic was in development in 2006 for DS and announced at E3 2007. But in 2008, the game was cancelled, probably because the trend was getting old. After this was cancelled, they tried to revive the engine for a mini game called Sonic7. But this one never came to be either.
No demo or gameplay footage is available online.