It's nice to see you here in the Community, Ryan.

Let me provide some information as to why your client's bank balance and QuickBooks balance are mismatched. 

Yes, it's possible that past incorrect reconciliations may cause the mismatch. I'd suggest further checking the previous reconciliations to locate any discrepancies. Once you've identified the issues, you can fix them accordingly.

Additionally, many other factors could cause the balances to be different. It's essential to investigate the root cause and make the necessary adjustments.

Here are the possible reasons why balances may not match:

• Outstanding transactions
• New transactions for a connected account
• Duplicate transactions
• Edited or deleted transactions
• Credit card (liability) account

Furthermore, it's worth noting that open invoices that still need to be collected post to the Accounts Receivable (A/R) account in the Chart of Accounts. This doesn't affect your client's bank and QuickBooks balances. Once the invoice is marked as paid, the amount flows into the designated income account.

For more information on accounts receivable, you may refer to this article: What is accounts receivable?

Once you have resolved the balance mismatch, I recommend reconciling your client's bank account every month to ensure that the balances correspond. You can find a detailed guide on how to do this here: Reconcile workflow.

Please feel free to reach out if you have any other questions about the balances in QuickBooks. I'll be around to lend a helping hand. Keep safe.