Sometimes a purchase of securities has no corresponding sale. In that case, the clearinghouse sends a DK notice to the purchaser's broker indicating that it can't match the transactions. Inaccurate trade symbols, price per share, or quantity transacted discrepancies. Bad handwriting. Fat fingers. And other likely human errors all contribute to the DK stamp of shame.
When this stamp happens, the brokerage has to return to the writers of that trade and, well, just figure out WTF happened.