[gp2000]

The bug is explained by Dave Theurer himself in this video: https://www.youtube.com/watch?v=41TbGi7u598&t=168

Like many games at the time the code in the ROMs had tamper protection. The basic idea is to detect if the ROM has been altered and if so assume it is a pirated copy and do something to cause the game to fail. Pirates could still make exact copies but they'd have to retain copyright notices and trademarks which makes them more vulnerable to detection and prosecution.

At the last minute Dave changed the position of some logos on screen but forgot to update the copy protection. When an illegal copy was detected the game would use some of the digits of the score obtained at the end of the game to zap bytes in memory if the overall score itself was within some range. This kind of random scrambling that only happens semi-randomly is better than copy protection that triggers deterministically. Imagine if the game did a checksum of its ROM and immediately said "Illegal Copy Detected - Program Halted". It would make it easier to find the copy protection routines and disable them.

So the first version that shipped thought it was an illegal copy and triggered the random scrambling. It so happens that sometimes the random scrambling would result in giving the player 40 credits.

There were other effects which players discovered and are listed here: https://forums.arcade-museum.com/archive/index.php/t-79058.h...