What happens, when you change a line item previously posted to Billable expenses > Customer to an inventory item, is that the subsidiary ledger for Customer is not cleared. In fact, if you change the account back to Billable expenses, the customer’s name is already there.
An example of this would be purchasing an item you thought would be billed directly to a customer rather than placed into your own inventory. Then, for whatever reason, you decide to put it into inventory and sell it from there.
I’m putting this into the bugs category.
Note: I have not tested this, but similar things may happen elsewhere in the program where subsidiary ledgers are involved.