Due to no obvious cause, a white "Space Invader" sometimes flies across the screen when you are approximately 60% through the game.
The English bootleg just says 'England' instead of the copyright message before the title screen.
TIPS AND TRICKS
Super Power-Up : Start a game, don't score any points and get killed as soon as possible. You restart with 1000 points, all power-ups & weapons.
Extra Super Power-Up : Start a game, don't score any points and survive until you've passed the first pond/fountain, which appears on the left hand side of screen. You restart with 180,000 points, all power-ups & weapons and are advanced much further in the game.
Helicopter : Start a game, get killed TWICE as soon as possible. You restart with a helicopter sitting on the first left hand side ledge which you can shoot for 170 points.