To Elaine's list let me add one more. If you create an invoice, debit memo or credit memo in AR and use a receivable account in the grid portion of the screen you will also have this issue. The reason is that the AR aging works at the document level, not the transaction level whereas the GL works at the transaction level. Consequently, if you create an AR document that both debits and credit a receivable account, the aging is only going to show this document once but the GL is going to show both sides of it (netting out to 0.00).
I see this when a user is trying to reclass something from one receivable GL account to another and gets cute and tries to do that in a single document or they create a debit or credit memo thinking about what receivable account is affected and incorrectly enters that receivable account down in the grid portion of the screen.
These can be a bit difficult to find but, if you are comfortable running SQL queries you could do something like the following:
select * from ARTran where trantype in ('IN','DM') and DRCR = 'C' and acct = 'xxxxxx' and perpost = yyyymm
select * from ARTran where trantype in ('CM') and DRCR = 'D' and acct = 'xxxxxx' and perpost = yyyymm
If you have either this situation, one of those queries will return something. The basis behind the queries is that an invoice or debit memo should never credit a receivable account and a credit memo should never debit a receivable account.