Teenage Mutant Ninja Turtles: The Hyperstone Heist/Hidden content
From Sega Retro
Stage select
On the title screen, press 
START . When starting a new game, a stage selection menu will appear[1].
"Friendly fire"
On the Konami logo screen, press . Now when starting a two-player game, the two players will be able to hit each other and cause damage.
This code was disabled in the NTSC-U version of the game for unknown reasons.
References
