I decompiled the original version of Spelunky made on GameMaker 8 back when that was the latest release and saved it as well as GM 8 somewhere so that I could compile it and play it if it ever got deleted. It was an amazing learning experience getting to look at the amazing Derek Yu's code and see how he used the engine, helped me understand more complex concepts and see what more optimized code should look like. Such a simple game but I'll never forget it and that experience.