In regards to making a foolish fish keeping mistake?

Mine is when I was younger I didn't know anything about the cycling process - amazingly my fish didn't really suffer for it! But luckily I later learned about it before getting more fish.

Ah yes, a lot of people are very misinformed on the cycling process (usually by pet store employees who tell people to just leave the tank running without fish for a week and then all is fine) or have never even heard of it before.

For me I've made a lot of elementary mistakes like not enough water changes, keeping fish in too small of tanks, not using a heater where I should've.