in all seriousness though, to answer the original question, I haven't had any huge mistakes. I am glad that I started slow - still a newb - and now that I am playing with my tank and reading and reading more, my ambitions are growing.
Take to heart what people say here . There are a lot of people who know what they are talking about. When I first started I had NO idea what I was getting into, but I was lucky it has all worked out so far (knock on wood).
When I get a bigger tank and really do it for real (ie, reef tank, good lighting, etc), I will base most of my purchases on advice from swf. so far so good, but my advice to another new hobbyist is to take it slow and learn as much as you can. It will make your tank better and save you money at the same time