Why doesn't the A/P subledger match the A/P general ledger account?

A common problem people face using Dynamics GP is that the total of the accounts payable module doesn't tie to the total of the accounts payable account(s) in the general ledger.
Here are some things to check:

1)   Consider the possibility that there are some "accounts payable accounts" that you have not included in your comparison between the a/p trial balance and the g/l.     The a/p trial balance report shows you every payable in a/p module that is open, regardless of which g/l account was credited.    When someone enters an a/p invoice in the a/p module, the credit entry defaults to your "normal a/p" account.   But someone might have entered an a/p transaction and changed the credit to some other account (such as accrued commissions, or other liabilities, or bonuses payable, etc...).    If you aren't including this "unusual" account in your reconciliation, your comparison between the a/p trial balance and the g/l will be off.
2)   Use SMARTLIST to check to see if someone booked a journal entry to the a/p account in the general ledger, thus bypassing the a/p subledger.
3)   Has someone accidently reversed the signs on an a/p transaction?
For example, if someone accidently changes the accounting on an a/p invoice (in the distributions window) to say:
             Accounts payable         $1,000
                      Travel expense                $1,000
rather than
             Travel expense              $1,000
                     Accounts payable               $1,000
you will have a $2,000 difference between the a/p subledger and the g/l balance.
4)   If you have any receiving transactions in the PO module, make sure that any "shipment only" receiving transactions are NOT creating a credit to the a/p account.
Most companies create a special account to record the liability for "received not invoiced" shipments.    If you aren't using such account, and you are insteading hitting your normal a/p account in the g/l, you will create a difference between the a/p subledger and the g/l because your a/p subledger will not know about any of the "received not invoiced" shipments.
5)  If all else fails, use SMARTLIST to search the g/l for the amount you are off.    Sometimes, you can get lucky.