Eek, SQL-Ledger has lost some stock

I seem to have found a gremlin.  I think the situation was 
this(sql-ledger 2.6.9):

- Created Purchase order for 5 items
- Used "receive goods" to note that they had arrived
- Noted (I think) that I now had 5 items in stock
- Sold two items with invoices
- Noted that the invoice for the 5 items had now arrived so I converted 
the purchase order to an invoice and then because I wanted to track 5 
serial numbers I changed the amount to 1 on the line item, then added in 
4 more line items the same. 
- Posted invoice,
- Just noticed (later) that I have -2 items in stock!!

All the invoices are there - I ran a parts report and I can see my 5 
individual items being bought and the two sales.  However, "On Hand" 
says -2?

What's the best way to fix this?  I tried reposting the invoice for the 
5 individual items (no joy)

How do I avoid causing this to occur again...?

Cheers

Ed W