The game crashes if the player selects Kempston joystick. Technically, there's a 'PUSH DE' in the joystick routine but no corresponding 'POP'. When the routine hits the 'RET' instruction, it incorrectly catapults to the value of 'DE' from the stack, thus crashing the game. The correction simply replaces 'PUSH DE' with a 'NOP'.
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