Stock cost became zero

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Hello all, 

We faced a bizarre scenario.

An agent was posting invoices and within a minute's difference, the stock cost became zero.

The quantity was already in negative but was carrying the previous stock cost until a credit note was posted and thereafter all cost became zero. This happed for only 1 stock item.

Nobody has changed the cost thru inventory journal nor adjustment - or rather  no have no trace of it.

Question:

1. Is there a way to track manual changes to cost?

2. What could have caused that stock cost to become zero?

Thanks for your replies.

Ash