Biggest mistake I ever made (this was when I was 9 and knew next to nothing) was having 15 common goldfish, 1 koi, 1 pumpkinseed, 2 bluegills, and 2 common plecos in a very poorly maintained 240 liter tank. There was a big crash less than 6 months after we set it up, and nothing survived. I didn't know enough to do proper maintenance either. Egad it was bad.
As for mistakes I made after having some idea of what I was doing, the worst (from 3 years ago) was trusting Big Al's recommendation that 3 yoyo/zebra loaches would be enough (it wasn't, they got very aggressive towards my properly-numbered group of 6 clown loaches). I was not comfortable at the time trusting sites like Loaches Online/MonsterFishKeepers/etc, which, looking back in hindsight, I should have been.
(Fortunately, that Big Al's has now upped its minimum loach recommendations, I am no longer afraid of online fish advice, and I will ensure to have both of those species in proper numbers when it comes time to purchase them this upcoming summer).