I remember a hotfix for this. The solution was an extra condition on a where clause to read from SalesParmTable, somewhere in a SalesFormLetter** class..
In Standard, there was only a where on ParmId, and the hotfix had an additional field. I don't precisely remember it..
You can either look for the hotfix, or try to find the Code where SalesParmTable is read...
I also think, the Scenario Sagar mentioned could be a potential result of this missing condition as well..