Accidentally dropping a piece on the wrong square and missed win
Plot twist: The "accidental dropping a piece on the wrong square" turns out to the the best move (better than the planned move).

It happens. But statistically speaking, it sort of averages out, so you opponents are doing that about as much as yourself.

Plot twist: The "accidental dropping a piece on the wrong square" turns out to the the best move (better than the planned move).
In a critical over-the-board Tournament game (last round, must-win situation) I blundered away one of the Pawns in front of my castled King. Just totally overlooked that my opponent could take it.
It turned out to be an awesome Pawn sacrifice. I won.
Eureka! Just found if you click on the piece you want to move and then just click on the square you want to move it too, you won't have to worry about dragging and dropping the piece on the wrong square by mistake!
Terrible feeling