Made a few mistakes as a kid, this was probably 18 years ago or so (26 now).
Started off with a 20g, and knew nothing about fish. Didn't overstock it too badly; started off with a couple of angelfish, some neon tetras, some cories, and a couple of those blasted chinese algae eaters (i still hate them till this day). I was smart enough to research the nitrogen cycle though, so my tank was cycled by the time it was stocked.
Then I discovered Petsmart.
They sold me six clown loaches.
Most of the neons died in the following 2 weeks, all the clown loaches got ich and only 3 survived (i still hate myself for that, they still are one of my favourite fish). I ALMOST made the mistake of using a copper containing chemical to treat them, but my LFS had some great employees that steered me in the right direction.
Realised what the problem was, guilted Dad into buying me a 55g for my birthday, promptly rehomed the fish into it (wound up giving the loaches that survived to my LFS).
Had a really strange occurrence with congo tetras though.
Left the remaining neons in the 20g, saw some congo tetras and a GBR at the LFS and bought them, figured they would be fine in the 20g with the neons.
The lights went off that night, and it was a bloodbath.
The seven remaining neons were torn apart by some very vehement congo tetras, and those tetras remained alone with that GBR until we moved away and gave them back to the LFS (we moved from Canada to Australia).
Most people learn once they **** up a few times, but not always, unfortunately.
Saw an ad on a local marketplace while browsing fish and tanks, some lady has had an angelfish in a 5g for its entire life.
It still managed to grow to mostly full size, but damn.
I can't afford to go around rescuing every fish (that damn barramundi is going to be trouble, I can see it now).