There are a few unused tracks in the game that are accessible from a hidden Music Menu. "newdream4" is different depending on if you're playing the US or EU version of the game. In the EU version, it sounds like an unfinished remix of worlds 2 and 4's music. The other, "sbr_mall_grager" is a very short tune that sounds very unfinished. It could possibly have been a piece of music associated with a boss.

There's a few unused messages hidden in the ROM that differ from the US and EU version. In the US, there are 3 messages of "Hello there!" whereas in the EU version they read:
"Wow Salem, so this is what the inside of a hex editor looks like..."
"Hello there!"
"Hey look! Someone ripped our game!!!"

There is also a large chunk of uncompiled code, and for some reason the file path to the developer's ZoneAlarm settings: "C:\PROGRAM FILES\ZONE LABS\ZONEALARM\ZONEALARM.E"