Nightmare Fuel (Joubutsu2000. The lyrics itself is about a man giving up and gaining peace before his death. Hear a woman speaking 'Trois', 'Sept', 'Cinq', 'Six', 'Quatr'? "3,7,5,6,4" in japanese sounds like 皆殺し, which means "kill everyone". That crackling noise near the end? It's the sound of an incinerator, like those you see in a mortuary.)
Tastes Like Diabetes (Once you reach the apex of a song, your little drum character starts dancing "The Monkey"... and it's absolutely adorable!)
That One Boss (Any song with a difficuty rating of 10 stars defines this trope.)
Actually, scrap that. While the 10 star ones are the end bosses and are nearly impossible, any song above 7 stars in Oni level will cause an acute onset of DS Throwing Syndrome.