1) Purchase Order-->Received Inventory . When Invoice is received instead of matching Invoice created Payable Transaction with same invoice no. PO which is has Inventory Receipt didn't matched with Invoice. PO Status is still show open.
2) Purchase Order created --> Receipt with Landed Cost . Invoice Matched without Landed Cost.
3) Purchase Order created-->Receipt with Landed Cost. matched Invoice for Receipt , AP Clerk forgot to matched / Apply Landed cost in Purchasing Invoice. After receiving separate Invoice for Landed Cost Expenses AP clerk created Payable Transaction for Landed Cost Invoice and paid.
4) Purchase Order -->Partial Receipt with Vendor Confirmation that he is not going to ship other Item. Instead of cancelling other Line Items in Receipt. AP Clerk Received Item --> Matched Invoice. As line Item for those PO is not cancelled PO status still show open.
All above mistake can cause Purchase Order status open and Accrued Purchase Account will show balance. Make sure to check when you will do edit PO it will create some GL Transaction for cancelled/closed PO Line Items.
For landed cost Invoiced adjustment which is not matched, to closed PO user must need to matched Landed Cost and create Credit Memo with proper GL Account and apply Landed Cost Invoice to avoid Voiding all payment and payable transactions.