Robot Carnival is the third boss of Sonic Heroes, occuring after the sixth level, BINGO Highway. Not a boss machine in the traditional style, the Carnival instead consists of an arena where waves of Eggman robots appear and the player must destroy them all to advance. When a group of robots is destroyed, a new group appears.
The first wave is usually ground soldiers. A few waves later there are some flying robots and near the end there are bombers. This "Carnival" of robots seems never-ending, however when playing as Chaotix it can be quite easy: just slaughter the robots with Vector and Team Blast.
A similar waves-of-enemies fight also appears as the game's sixth boss after Mystic Mansion: Robot Storm.