For me, it was in an arcade located in one of the local shopping malls near my house. It was somewhere in the neighborhood of 1993/1994, and I was having my parents take me to the arcades for some Street Fighter/Mortal Kombat II action every weekend. One day, as I was waiting for my turn to play the newly released MK2, I saw the arcade operator (employee) busting out crazy Fatalities and Friendships that nobody else had seen yet. When asked how he did them, he pulled out a sheet of paper with a bunch of text printed on it. The paper was the old kind that you'd use on a dot-matrix printer, with the two perforated edges.
I didn't get to look at it closely, so I asked my dad about it as we left the arcade that day. My dad, being a software developer, told me that it was probably something from the "Web." I had never heard of such a thing, and even after my dad briefly described it to me I wasn't fully comprehending the idea.
The next weekend I went back that same arcade, only now there was hardly anyone there except for the operator from the week before. I decided to go ask him about that sheet of paper, and he brought me to the office in the back of the arcade. He had what I believe was a newsgroup posting open on the computer, and it listed all of the discovered moves/fatalities for nearly every MK2 character. There were still some blanks for the ones that had yet to be found, but I assume people in the newsgroups were updating with new moves as they came in.
It was right then and there, seeing all of those hidden commands listed out on the screen, that I knew I had to find a way to access the Internet.
It actually wasn't too long after that (maybe 6 months or so), that my dad brought home a computer and a 56k modem. By this time, it was probably late 1994 or so. Once we got everything set up with the ISP, I launched Netscape Navigator and embarked on a journey of insatiable information-lust. My dad recommended a website called InfoSeek, which would let me search for whatever topics I pleased. I typed in "mortal kombat" and was greeted with a listing of various MK fan-sites. I then did the same for the recently released Killer Instinct, and it retrieved similar results. It instantly felt like the world had opened up to me. The flow of information into my life was no longer confined to my family and friends.
The irony is, the Internet essentially killed what was so great about early 90's fighting games in the arcades. Discovering secrets and moves through word-of-mouth was a big part of the appeal, and that vanished completely once the WWW exploded into mainstream use. In a way, that piece of dot-matrix printer paper signaled the end for my childhood obsession just as it had signaled a new beginning for information sharing.
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I'm sure a lot of you were exposed to the 'net far before I was, and maybe some of the youngin's around here will have laughably recent stories to share (little rascals...). I look forward to hearing all of them. Either way, we have been pretty lucky to have witnessed the birth and growth of something that has changed the way that we live, worldwide.