Since everyone on here are pretty experienced fish keepers, I figured this would be fun. Share your "I was a stupid newbie" story. It's humbling 
I'll start:
I first started out with a 10 gal, one of those kits that you can buy at Wal-Mart for $30. Anyways, I brought it home, filled it with tap water, no heater, in the dead of winter. Next day, I went to Petsmart, told them that I had a brand new 10-gal that had been set up since the night before, and I came home with 6 tiger barbs, a columbian cat, iridescent shark, bala shark, and a paradise gourami. Well, the paradise gourami and bala shark died right away. The rest of them miraculously survived, and the catfish even survived a battle with ich. Well, over time, the bala sharks started disappearing one after another. I kept checking the insides of the filter, around the room near the tank, etc., puzzled as to where they could have gone... Well, the rest is history.

I'll start:
I first started out with a 10 gal, one of those kits that you can buy at Wal-Mart for $30. Anyways, I brought it home, filled it with tap water, no heater, in the dead of winter. Next day, I went to Petsmart, told them that I had a brand new 10-gal that had been set up since the night before, and I came home with 6 tiger barbs, a columbian cat, iridescent shark, bala shark, and a paradise gourami. Well, the paradise gourami and bala shark died right away. The rest of them miraculously survived, and the catfish even survived a battle with ich. Well, over time, the bala sharks started disappearing one after another. I kept checking the insides of the filter, around the room near the tank, etc., puzzled as to where they could have gone... Well, the rest is history.