Can I "undo" a pairing of a transaction with a bank transaction that was in error? Where do I find that?

Let's get you out of this situation so you can continue itemizing your bank transactions in QuickBooks Online (QBO), sbeansmby247. 

Since you've already categorized those transactions in QBO, we can manually Undo them from the Categorized tab. I'll show you how:

1. Go to Banking from the left menu and click the Banking tab. 
2. Choose the tile for the bank or credit card account you downloaded the transaction.
3. From the Categorized tab, find the transaction you want to unmatch.
4. Click Undo from the Action column.

I've added a screenshot for your reference:

In addition, you can refer to this article to learn more about handling your transactions inside QBO: Categorize and match online bank transactions in QuickBooks Online.

Aside from that, I've also got you this handy article about reconciling an account: Reconcile an account in QuickBooks Online.

Let me know if you need further assistance managing bank transactions inside QuickBooks. I've got you covered, and I'll help you out again. Stay safe!