OK, let me recite my experience, which in my opinion does take the cake when it comes to stupidity.
When I was employed as a research scientist (that was a long time ago), we had a summer student, he was a Physics major. Somehow we acquired an empty tank, and he decided to make it into an aquarium. I told him to go ahead.
So he filled it up with water, added a few fish and for a few days everything was great. Then the fish died one by one. He bought more fish, the same thing again. Somebody suggested to him that perhaps water may have some algae etc, growing in it, which may be harmful to the fish. So he bought a few snails along with the fish, with the hope that snails will eat up whatever was harming the fish.
In a couple of days all the snails split up and died. All the fish also died. During coffee break we were discussing what may be killing the fish. Somebody again mentioned water; there may be something wrong with the water.
“There is nothing wrong with the water”, the student replied heatedly. “In fact, just to make sure that water is absolutely pure, I filled the tank with distilled water.”
Thud! Only a physicist would make a blunder like that.
That anecdote sounds like someone else's stupidy, I think the gist of the thread was supposed to be about your own stupidity.