If you've purged transactions and then try to undo a reconciliation

When you purge transactions, transactions are deleted from your company file. If you try to undo a reconciliation in which some of the transactions have been purged, you will receive an alert message.

We strongly recommend that you don't undo a reconciliation in which some of the transactions have been purged. Transactions that were necessary to reconcile the account the first time were deleted from the file during the purge process. Because of the missing transactions, if you undo the reconciliation, you won't be able to reconcile the account again.