Strange Navision Problem

Last week I encountered with the following bizarre problem. In purchase orders I made a post of receipt and immediately I made a post of invoice. Because I forgot to put the related document no. of vendor the invoice throws an error (thats normal). Navision made a rollback of the transaction but the strange thing was that it rollback the receipt as well. At first I thought it was something wrong with our custom code but after an extensive examination I found that the problem was that after the post of receipt the transaction was not closed and when the invoice failed to post the rollback was made to the point before the post of receipt. I tried the same thing but I closed the purchase order form after the receipt, I opened it and then I made the invoice. Now it worked! For the record I used Navision 5.0 sp1 with sql server 2008 developer edition on windows xp sp3. In order to bypass the problem, I made an fbk backup and restore to a new database (the sql backup replicated the problem)