Arnie Savage: Combat Commando

Demo

Commodore Format magazine gave the C64 version a controversially high score, and also included a demo on their covertape, featuring the full first level. However, the game features a bug making it possible to skip level one relatively easily - and this also works on the demo, which had the game's entire code beyond an intended block at the end of level one, meaning that they had effectively given the entire game away (if you knew exactly what you were doing).