Let's see.....
I had a 40 gal acrylic, then graduated to a glass one. The stand I had it on wasn't level, sturdy, or flat, but the acrylic seemed to do fine on it, so I set up the glass one in the same place. The next morning I had 40 gallons of water on the floor, luckily fish were still in a holding tank. There was a crack through the bottom of the tank. I bought a REAL tank stand the next day, and the lfs replaced my tank (even though it was my obviously fault).
I made a DIY CO2 system (yeast+sugar Rxn) with a ladder diffuser, and neglected th "optional" check valve. I came home one day with the bottom of my aquarium stand FULL of water. Like a solid 2 inches. Good thing is I found out the base of my stand holds water, hence no carpet damage. I bought a check valve the next day.
I tried to save a few minutes by not reading the gravel syphon instructions. My dad always started a syphon with his mouth, so I did too. I ended up with a mouthful of dirty aquarium water. YUCK! Then I read the directions and haven't had a taste of fish waste since.
I was doing a wc shortly after buying my first powerhead. So naturally, I forgot all about the fact that it is NEVER to be plugged in while not completely submerged in the water. I noticed the powerhead (turned on while out of the water) and thought I just stupidly broke a $50 piece of equipment. I unpluged it then resubmerged it and got all the air out and it worked again! YAY! Lucky me. I also relocated the powerhead so it is below my usual WC level.
I've also skipped out on warnings for some medications and killed off my crustaceans.
Put a 6" bichir in a community tank (no fishies left in the tank by morning).
Put my hand in my tank without washing it first. COMPLETELY forgot I had sunscreen on...... no more fish left in that tank. Now I ALWAYS wash my hands before doing tank maintenance.
AND now the worst...... (did not happen to me, but to a friend)
My friend's mom had a 125 w/ oscars. She was watching my friend's son (a three year old). They both went down for a nap. The toddler woke up before his grandma, found a bar of soap, and somehow managed to put the bar of soap in the aqaurium. That was a stressful day lol. The tank was fine though...... luckily.