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XjyR TIL The aliens in Space Invaders were supposed to stay the same speed, but as the number of enemies fell it became easier to render, increasing the speed and adding a difficulty curve.
OJpX TIL Grand Theft Auto was originally a racing game called Race'n'Chase, however a glitch made the police cars go crazy, ramming the player. This glitch was so popular with testers that they rebuilt the game around it, creating GTA.