I noticed in a check register (Account B) that a check written from one account (Account B) to another (Account A) had been entered twice. Same amount, same date. This was back in July. I deleted one of them. I'm pretty sure that the one I deleted had not cleared, so I thought I was OK.
When I went to finish up reconciling Account A for December 2020, my beginning balance was off by the exact amount of that check.
Thank you for following up with us here on the Community page. I can help you with fixing the discrepancy in your reconciliation.
Yes, you're right about undoing the past reconciliation in QuickBooks Desktop (QBDT). Moreover, QBDT 2017 must not be an issue with the process. If you wish to, you can upgrade your QuickBooks versions to get the latest features available. You can open these articles for your references:
Since you're still getting an incorrect balance, you can proceed with running a discrepancy report or checking your audit trail to find the transactions causing the discrepancy.
Go to the Reports tab, then select Banking.
Click on Reconciliation Discrepancy.
Choose the appropriate Account, then select OK. You now have a list of transactions that were changed since the last reconciliation. Use this report to identify the transaction/s causing the issue.
If you find a discrepancy, note the transaction date and the Entered/Last Modified, which will tell you when the change occurred.
To fix the discrepancy, you have to re-enter or edit the transactions. You can undo your previous reconciliation and redo it or ignore the discrepancy and let QuickBooks enter an offsetting adjustment. For your guide, check out the instructions under To fix an incorrect balance section.