It's dangerous, it's devious, it's Xevious. Atari was planning on porting the popular arcade shooter Xevious to the 2600, despite the great difficulty involved. But sadly like so many 2600 arcade conversions before it, Xevious was not be. Another victim of the management shake up at Atari.
More information: http://www.atariprotos.com/2600/software/xevious/xevious.htm