Two instances come to mind. I accidentally reversed the input and output hoses on my Fluval FX6 canister and 5 of my beautiful ryukin were sucked into the output and ground up in the impeller. The other was a water changing accident where one of my fish swam up the tube and got it's head stuck in the uptake tube. It's head was exposed to extreme negative pressure and sucked parts of it's head off. It happened so quickly. The other way I've lost goldfish is when they get determined to get through a space that could not possibly accommodate them. All they need to do is back up 1/4" to get out of the situation but instead they choose to swim forward until they exhaust/stress themselves to death. I suspect inbreeding to be the source of this self destructive behavior since it strikes me as being supremely stupid. Everyone has seen those hollow ceramic decorative bridges. Those are deadly to goldfish. I took all rocks and vases out of my tank and have the gravel peppered with a variety of acrylic spheres for ornamentation. The largest being 4" in diameter. Even those I need to keep apart from each other, otherwise it makes a space for them to get their nose into.
Glenn
Glenn