Usually the reason for that is that a step was missed in the sequence for closing a trade/sale, but Transaction Completed was clicked.
I have read, though, that there has been an occasional glitch, but I'm more apt to believe the former is the reason.
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“Aslan didn't tell Pole what would happen. He only told her what to do. That fellow will be the death of us once he's up, I shouldn't wonder. But that doesn't let us off following the signs.” - Puddleglum in The Silver Chair by C. S. Lewis