(sorry, i've got kind of a lot of them. I've done some pretty stupid things over the years)
I've made the mistake of trying to keep a goldfish in a bowl, as well as bettas.
When I was about 8, I went out and dug a hole in the middle of the lawn, cut open a black trash bag and lined the hole with it, filled it with water and threw some gravel in and my whole family actually tried to keep 3 normal comet goldfish in this 1 foot deep, 2x2 hole next to our porch. They actually lived for a few weeks until raccoons figured out what was going on.. We stuck a big grate over the top of it (Looked like some kind of fish dungeon) but the last one didn't live much longer. This was early fall, also.. I'm not really sure how I thought that was going to work.
My first actual fish tank was a 20H and I kept an angel fish, dyed fish, neon tetras, guppies, and who knows how many different tetras types in there and my mom just kept replacing them if they got eaten or died.
I just thought fish didn't live very long. I also forgot to turn my light on many days and often didn't even feed my fish.. I'd go and finally turn the light on and everyone would have their fins missing.
When I cleaned my tanks, I would take every plastic plant and decoration out, scrub them down, scrub the filter cartridge and vacuum the gravel all at once. One of my biggest regrets is one time during one of these bacteria massacres, I had put all of the plants in a big pot while I was vacuuming and afterwards I was putting them back in one by one and when I got to the bottom of the pot I found a little blackline raspbora. I still don't know if he was already dead when I took the plant out but I really hope so.. I feel really bad just thinking about it.
Not all that long ago, I went to petsmart with the plan of keeping some kuhli loaches and ghost shrimp in a 10 gallon tank and when I actually saw how tiny and invisible the shrimp were, I decided against it. The lady convinced me to stick a leopard ctenopoma and rainbow shark in there, and told me they would be just fine and wouldn't grow much bigger, would only eat flakes etc. She also tried to sell me 2 electric blue rams and 2 julii cories and the only reason I didn't buy them is because they were too expensive. Went home and did research and now I realize she had no idea what she was talking about. Now i've got the two fish in my 75 gallon tank, where both have already grown considerably.
I put my screen top on my Green Spotted Puffer's tank at a bad angle and the filter I had was touching it, didn't think it was an issue until I woke up to crazy zapping and popping noises and saw that my tank was half empty, the floor was flooded and an air pump shorted out. The water in the back of the filter was running across the back of the screen top and down my airline tubing. Had a power strip get water logged and it somehow still works, too. I went ahead and did this exact same thing AGAIN, not having learned my lesson, and once again flooded the floor. (Two times is enough, thanks.. Let's put that other top on.) My entire room still smells like the ocean.
Not exactly fish, but when I was little I found a huge blob of frog spawn in a puddle behind my house and we set up a tank for them in the freezing garage with no filter and somehow expected them all to live.. They actually hatched but we started finding these tiny little worms in the water which we finally realized were mosquito larvae, and around that time each of the tadpoles vanished without a trace.
I remember as a toddler, I put a chair up next to my mom's 90g aquarium, netted out a pretty little orange swordtail, ran across the house with it and plopped it down on the carpet in my room, happy as can be. It flopped around for about 15 seconds until my mom found out what I was doing and saved it. (I'm still not sure what all that was about)
This isn't something I did, but I figure I may as well share. When I was a baby, my grandfather came over our house with a few friends and while my father was in a different room, my grandpa pulled out one of my mom's $50 saltwater fish, smacked our dog across the face with it and threw it back in. My dad came back in the room and they were all laughing and wouldn't tell him why for a while. The fish lived, but my grandfather wasn't exactly welcome near the fish anymore.
I still loved that fish, which turned out to be a Mystus leucophasis and was subsequently christened Dr Death.
