Blitz Player rf(Robert Fischer diehard fan probably) usually crushes me, but today he blundered trying to castle Queenside. I don't have incontrovertible proof, but with the center wide open he moved his Queen to d1 rather than TRIPLE O.
Do you see the mate in 2 in the position below with Black to move? Rc1 attracts Whites rook to that square and then mate at d2 for the shouldered White Monarch. Note that Epaulette Mates would NOT BE THING if two pieces could occupy same square!
So, Yup attempted castling is the most common blunder. I guess some people just can not get used to King being able to move two squares.