Online balance and ending balances don't match in my register, where's the problem?

Something I started a long time ago was entering the month ending balance from the FI for each account (that match my statements) into the Notes column for each month. One caveat is to be sure your set your transactions sort to > Sort by date / Order entered, and also use that sort order for verifying your closing date balances if you go back to look for where the error is.

The first thing that should be checked is the opening balance though.

There are times when your online balance and ending balance don't match. However, if you have the correct balance annotated in the notes column, you can easily scroll through your transactions until you find the transaction for the closing date that does not match your balance that you verified previously and annotated in the notes column. Boom, you found the month that has an error. Correct any error for the month (Quicken vs FI statement) then keep going from that point to the next earliest and check again. After every correction, look at your online balance vs ending balance is correct. This has saved me MANY hours looking for a rogue transaction that has messed up my totals (online vs ending) in my register.