3. Speed Up - Space Invaders (1978)
http://www.youtube.com/watch?v=437Ld_rKM2s Along with the likes of Pac Man and Tetris, Space Invaders is considered to be one of the essential retro classics, though you might be surprised to learn (once again) that much of its success is owed to a glitch. If you've ever played Invaders, you'll know that - with each passing level - your alien foes becomes faster and faster, thus making the game increasingly harder, depending on how long you play for. You might recognise this as a simple "difficulty" level-based structure. Most games have them, right? It's kind of the point. Not at first, though, as Space Invaders originally wanted you to play the same level over and over again at the same speed. Forever. Without anything different happening. It was only down to a memory glitch - one which forced the game to get faster and faster the longer it went on - that Space Invaders was granted the frantic gameplay that we all know and love. So, yes, my advice to budding game developers is: don't take out the glitches - they'll make you a fortune one day.