Door games are, in a way, what got me interested in reverse engineering. Being a teenager running a BBS for some friends in school, I didn't have any money to "register" the games (and was lucky that my parents ponied up the money for the extra phone line). I had some experience w/ 6502 assembly and Pascal (all self-learned), got a copy of a debugger (Soft-Ice?) and set to work figuring out how to "crack" the registration systems.

Eventually, my BBS got shut down because I was dialing into other BBS's all over the country to spread my "patches" (and racking up outrageous phone bills). :(