Last Months Closing balance is NOT the opening balance for this month??? WTF?

Yes reconciliation discrepancies can be a bitch to find - under any system.

When you commence the 'reconcile' process QB shows a 'prior reconciled balance' - if this amount is different than the actual reconciliation you did last month then something has changed.  But that 'change' may not be a transaction amount - any change to a reconciled entry (even in the notes) can cause it to become 'uncleared', and that changes the 'prior reconciled balance'.

Here one way to fix the problem: start the reconcile process, but put in a date that is one day only after the last reconciliation date, and for the closing balance enter the reconciled balance  from last month - and 'hide transactions after the statement date'. Now 'clear' anything that showed as cleared on the last report and 'Reconcile' - this gets you back to square one.  

Then start the real next reconciliation.

Still cant find the discrepancy? A reconciled transaction may have deleted, or been redated into the future.

RE: How is is even POSSIBLE that the opening balance for the reconcile isn't the closing balance of last month.

This happens when someone changes the Amount on a reconciled transaction, adds a new reconciled transaction in the register, or deletes or voids a reconciled transaction.

This is because the opening reconciled balance is calculated and is equal to the sum of all prior reconciled transaction in the account.

To find out what transactions were edited in the account, or deleted,  since the last time you reconciled the account, use the Audit Trail report and filter it for the suspect account.