We have had a situation where two completely separate transactions (with distinct numbers) have been merged together. See 216309 and 216961. These were sent together by DHL, but when sent 216309's details were merged into 216961. When I credited the invoice, the credit came up as AB instead of AA, which suggests that HLMS is seeing 216961 as an "additional parts" transaction. I have sent some print screens and the incorrect invoice and the delivery notes to your email address (as I can't work out how to attach here!).
There is no way that this is human error. Bug? Thoughts?