The "Groundhog (Beat Juggle)" by NOISA, mixed by Scrath Perverts, is a song featured in DJ Hero and DJ Hero Mobile mixed with "Ace of Spades". It is generally considered the hardest mix in the game, due to the fact that it puts every feature in the game (cross-fading, directional scratching, etc.) to an extremely difficult test. The mix also features numerous off-beat portions that only add to the difficulty.
The song is split up into several sections, which themselves are split into two categories: "tests," and "rests."
There are a total of six tests, each one more difficult than the last. In between each test is a rest, which consists mainly of minimal notes and difficulty, allowing the player to prepare for the next test.
The song is almost infamous due to a sarcastic British voice that talks to the player throughout the entire song. He mostly taunts the player, but also sets them up for the increasingly difficult sections of the song.