The Tsunami Bomb song, "The Invasion From Within", was omitted from the game due to Atlus still holding the rights to the use of the song and the title now being self-published by Nippon Ichi themselves. The Invasion From Within was originally played during the fights against Baal, Prier and the first battle against Maderas.
Contributed by Dazz
The code to unlock Etna Mode in the PSP version of the original Disgaea game was officially released through the Disgaea 2: Cursed Memories manga series about a year after the PSP game's release.
Contributed by Mass Distraction