Hmm, I've been relatively lucky so far but here's what I got:
When I first setup the tank I wanted to practice with my gravel vac to make sure I wouldn't do something stupid (like move the hose out of the bucket when looking at the gravel in the tank). I thought it through carefully and had the plan perfect in my mind. When I went to start, the FIRST pump of the bulb on the vac got the water started, but it also was at the perfect angle that the bulb itself came off the tubing. I was left holding a syphoning line that was now outside of the bucket, and in the 1/2 second it took to realize I dumped about 1/2 gallon or so water on the hardwood floor and carpet. That one really peeved me.
The other time I was using my diatom filter for the first time. Surprisingly it did it without fault. About 1/2 way through the run I accidentally lifted the intake out of the water for a second which sucked up a lot of air. It doesn't work so well when air is in the filter so, not thinking, I shut off the filter and turned it upside down as the directions say to prime the filter before use (it creates a vacuum that forces all the air out). Well guess what? When the filter isn't keeping suction, the powder that acts as the filter no longer stays on the mesh bag. I basically syphoned 2 cups of diatom powder right into the tank after it was basically done filtering. I spent the next HOUR blasting the powder off the leaves (it looked like Christmas in there). Not fun.
Where I think my big problems will come in the future:
-I do 5 gallon water changes with a bucket and have to walk back and forth between the bathroom and the office where the tanks are. I have yet to drop the bucket or bump it really bad (a cup or 2 here and there occasionally), but I know its bound to happen at some point. I honestly think the bucket itself might break which would REALLY tick me off since it wouldn't technically be my fault.
-My 8-month old daughter LOVES the fishtanks and can be very quick and powerful. I like to hold her in front of the tank and she gets all excited and bounces around. One time I was holding her around her hips and thinking she was far enough away, but she bent in 1/2 (with her head through her legs) and slapped the tank with both hands. The whole thing rocked back and forth and I was sure it was going to come down. This is my 20 gallon high tank that is on a metal stand btw, so its not as stable as a desk setup where the weight is supported along the entire base. I wish I would have gotten one like that!