Me, personally, a text adventure game filled with bugs that I did not know how to fix. (I realise only decades later that the index into array I was using to store the location references was probably incorrectly calculated when I moved sometimes.)
I learned a lot of programming from books like these (official links, not pirated):
There were more (one had a game called "Rats" and from the description I thought it would be a 3D game, but alas I never got it entered properly and even if I did, I realise now that it probably wasn't 3d rendered).