This code was discovered by JLukas of The Cutting Room Floor.

On the title screen, press Start to bring up the "START, NORMAL/REVERSE" menu and then enter the following code: Select, Select, A, B, Select, Select, X, Y, Select, Select, L, R, Up.

If done correctly, the title will change from yellow to red. When you begin playing the game, you'll notice that the backgrounds and objects are doubled. You can turn it off and on by pressing Select.

The best explanation for the presence of such an odd graphical effect is that HyperZone was originally planned to be played while wearing 3D Red/Blue glasses, a la Square's NES game Rad Racer, using anaglyph techology. When Select is pressed while playing Rad Racer it will activate the game's "3D Mode", as seen in this video demonstration:

While the objects in HyperZone were doubled, just as they were in Rad Racer, the Red/Blue colors were never added to them, thus making the effect incomplete. This goes a long way towards explaining both why HyperZone was made in the first place and why it feels rather unfinished. Hal Laboratory apparently wanted to create a 3D game, but for reasons unknown, could not go through with it. My guess is that either the game was made short and easy on purpose so that you wouldn't have to wear the glasses for too long, or development stopped when the 3D idea was dropped and the game was hurried to completion.

   

I also have to wonder if this is the reason that Hal Laboratory developed a 1st-person view RPG for the Super NES, Arcana (Card Master in Japan). Was the decision to have the game be in 1st-person view made with the idea of applying the 3D effect to it, too? Can you imagine playing a whole RPG in 3D? It would have been like the Avatar of its day!

Hold L and R and press Start on the title screen to bring up a Sound Test. Here you can test any of the game's 13 songs and 21 sound effects.

Interesting to note that the Halken Logo music is about twice as long as what gets played for the few seconds the company logo is displayed after starting up the game. The entire song can be heard in the sound test.

After you beat a boss, you'll have a few moments where you're still in control of your ship before the stage ends. Tap the brake button and try to get the Speedometer to read 333 just as the stage ends and the "AREA CLEAR" message appears. If the Speedometer is exactly 333 when "AREA CLEAR" appears, you'll earn an extra life.

Easter egg discovered by JLukas.