In the outstanding and transactions overviews, a zero-sum invoice doesn't show up. In my case, I credited a customer in one line for the same amount I charged in the second. Although the invoice may not be considered 'outstanding', the details transactions overview (open and closed) doesn't show the invoice either.
Outstanding should not show these. Does the ar/ap transactions summary report show them when you select closed?
1.3 only. A similar problem caused by an unrelated issue was fixed in 1.4
svn 5591