QB Issue Resolution:
For your year version, with your documents in hand for the last reconcile manually clear the check marks in the register. The undo reconcile and hide transactions after statement date were added in the 2013 version.
Resolution for Issue 'How do I undo a previous reconciliation on QB for Mac 2012?' available: Yes (Solved).
Source: Intuit Community forum.
How do I undo a previous reconciliation on QB for Mac 2012?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.Support for Quickbooks for Mac
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