Track #34 on the NSF is unused, although it can be heard in the game's hidden sound test. To access the sound test, start the game and lose all of your lives. Hold A and B as the screen says, "Game Over". The screen will go blank. Let go of A and B and the Sound Test will appear. The hidden track is #33 in the Sound Test.
NSF Source: Zophar.net
There are three unused stages in Zanac: Area 0, Area 13, and Area 14. All of these stages have scrambled background graphics, loop endlessly, and are devoid of the numbered weapon powerups.
Area 0 uses Area 12's music and appears to be either a memory filler stage or possibly a beta testing stage. It seems like it could have been used to test enemy patterns, and in particular, the game's AI system and the effects on it caused by shooting or not shooting the Sart enemies (seen in the screenshot, to the left of the Zanac ship) that lower or raise the AI. Use Stage Modifier 0091 000 to access Area 0.
Area 13 uses the ending credits music and is even more scrambled than Area 0. Use Stage Modifier 0091 013 to access Area 13.
Area 14 has no music and a lot of invisible ground targets. Shooting the targets will cause the game to crash. Use Stage Modifier 0091 014 to access Area 14.
The existence of Areas 13 and 14 suggest that the game was originally planned to have 14 levels, but was shortened to 12.
Code Source: GSHI