I'd love to know the details of that bug. All I've been able to find is this quote which doesn't really explain things:
The fact you can earn 40 free games with a certain score was the fault of developer Dave Theurer himself. He had created a special security code to protect against piracy which checked the placement of different objects. If the objects were not in the correct place, the game would shut down. Before the game was shipped, however, Theurer, who would fuss over minute details, noticed an Atari logo was off-center. He adjusted it slightly. This small change caused the code to malfunction and the player to earn 40 free credits if a certain score was reached.
Also, I seem to recall the bug makes an appearance in the plot of Ready Player One.