This isn't my FAIL, but my one my Dad made:
We had a family friend who had a neat 76 Dodge Power Wagon with a 400 BB and a very well built steel flatbed. Rocky had it built up by my dad and the thing was awesome. Not long after it was finished, it developed a rattle noise coming from somewhere inside the engine. Dad couldn't figure it out and the decision was made to tear the top end down to find out what it was. Before that could happen, the engine started running very poorly, smoking, sputtering and dying.
Rocky told me his ex wife had actually put a curse on his truck and he believed it was best he get rid of it after bad things started happening to it.
He simply gave the truck to me!
I towed it to my place and started checking things out. After confirming spark I checked for fuel. It was getting fuel, but something about the look of the gas coming out of the accelerator pump nozzles made me think there was water in the fuel.
After pulling the top off the Thermo Quad carb I saw mostly water in the bowl.
Winter was coming so I let the truck sit until the temps went below freezing. I pulled the fuel tank, pump filter and carb then dumped the fuel out. I brought the parts into the garage and let them thaw out. When the fuel tank thawed, about 12 gallons of water came out of it.
I found out later that his son had stolen fuel from his dad's truck and replaced it with water so the gauge read correctly.
Having solved that issue, the truck ran great...except for that mysterious rattle.
I pulled the intake manifold and found a Bonney 7/8" impact socket laying in the lifter valley. It's black oxide finish was pretty well silver from all the bouncing around.
I went over to Dad, showed him the socket and asked with a grin: "So, you been missing a socket for awhile?"
Jim