Free Galaga
The classic 80s arcade game, Galaga, has a really interesting bug. Like most arcade games, it has some looped video of gameplay for you to watch while deciding which game to play. What’s different about Galaga is there is actually a way to control the gameplay even without paying for the game! At one point, the ship that you would control during the game, which I will assume you’re all familiar with, is about to get captured by an enemy ship. However, if at that time you fire the gun and blow that enemy up, it unlocks control of the ship and you can get a couple minutes of free gameplay! This big was actually unintentional by makers of the game, but was left in the game when it was rereleased decades later.
If you care to see the code behind it, this link explains the bug and how to fix it:
