mistake #1 - Not reading Mike Paletta's book, nor discovering this message board, before I set up my tank the first time.
mistake #2 - Using crushed shells as substrate - live sand being much more beneficial to the aquarium, plus enabling you to keep "burrowing" species.
mistake #3 - Being uneducated enough to trust my LFS when my new 26 gal tank was 2 weeks old and he sold me a yellow tang and a coral beauty angel, when I already had 2 Percula Clowns in the tank. Tangs need an established 75 gal or larger tank. angels are not an easy species for a beginner to care for.
mistake #4 - believing the LFS when a week later my tang and angel died (clowns were doing fine), and he said "it happens, fish die".
mistake #5 - ordering another yellow tang, a 4-stripe damsel, a royal gramma, and a chocolate chip starfish over the internet, and acclaimating and adding all 4 fish into the now 4 week old new tank, because I thought "since my clowns are doing fine, my tank must be ready for all those fish". They all died off, including the clowns.
It is horrible to make these mistakes, but I have certainly learned from them, and I hope you do as well. My new resurrected tank is now one week with live sand, live rock is going in soon, I have added a protein skimmer and powerhead to supplement my "weak" powerfilter, and in another month I will consider adding a cleaner / sand stirring crew and my first fish, if the water checks out to have cycled completely.
