Unreconciled items warning on matter completion

This message is purely informative and is advising that there are bank reconciliation items that are yet to be reconciled.
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You can tell which reconcilable items are reconciled quickly by looking for an asterisk (*) next to the transaction type code, on the Type column on the face of a Ledger Card. Also if you drill into a posting there is a column that shows the reconciliation report a transaction is in, if it's empty, then it has not been reconciled.
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If you are happy to do so, just acknowledge the message by clicking Yes, The matter will then close, and you will still be able to reconcile the items once they pass through the bank account.