When a profit and loss statement looks wrong for no obvious reason, inventory is usually the cause. Cost of goods sold is the largest expense line in most product businesses and the only one calculated rather than recorded, which means every inventory error arrives on the income statement wearing a disguise. These eight are the ones that show up most often, and each has a distinct signature on the statement.
1. Expensing inventory when you pay for it
What it looks like on the statement: profit that swings wildly month to month with no matching change in sales.
Inventory is an asset until the unit sells. Sellers who book the purchase order as an expense at payment get a loss in every restock month and an inflated profit in every month they do not buy. The pattern tracks the buying calendar rather than the business.
This is the single most common error in early-stage product businesses and it makes the statement unusable for any decision. Nothing else on the list matters until this one is fixed.
2. Using invoice cost instead of landed cost
Signature: gross margin that looks consistently better than the cash in the bank suggests.
Landed cost includes freight, duty, customs brokerage, and prep. On imported goods those additions frequently run 20 to 30 percent above the supplier invoice price. Leaving them out understates cost of goods sold, overstates gross margin, and overstates the value of inventory sitting on the balance sheet.
The costs do eventually hit the books, usually dumped into a freight or shipping expense account below gross profit. So total profit is roughly right while gross margin is badly wrong, which is worse than being wrong in both places, because gross margin is what pricing decisions are built on.
3. Ignoring inventory held at fulfillment centers
Signature: a balance sheet inventory figure far below what you know you own.
Units sitting in a marketplace fulfillment network are still yours. So are units at a 3PL or prep center. Sellers who count only what is physically in their own space understate inventory and overstate cost of goods sold, because units that never sold get treated as though they did.
Amazon publishes inventory reporting through Seller Central, documented in its inventory reports reference, and those quantities need to reach the count.
4. Leaving goods in transit off the books
Signature: inventory that drops sharply at period end and recovers the following month.
Ownership of a shipment transfers according to the terms on the purchase order. Under FOB origin you own the container from the moment it leaves the supplier, which means a shipment on the water at period close belongs on your balance sheet.
For a seller with a $60,000 container in transit every quarter, omitting it understates assets by that amount at every period edge, which distorts any ratio a lender looks at.
5. Never writing down dead stock
Signature: inventory value that only ever grows, and a gross margin that deteriorates without explanation when the stock eventually moves.
Inventory is carried at the lower of cost or market. Units that cannot sell for what they cost have to come down. Sellers who roll obsolete stock forward at full cost every period are carrying an asset that does not exist, and the correction arrives all at once, usually in a liquidation quarter where margin collapses and nobody can say why.
A 365-day no-sale trigger is a reasonable review threshold. Not everything past it needs writing down, but everything past it needs looking at.
6. Recording reimbursements as sales
Signature: revenue that exceeds what your order volume supports, and gross margin that looks unusually good in months with heavy warehouse problems.
Marketplace reimbursements for lost or damaged inventory are not sales. Two entries are needed: the lost units come out of inventory as a write-off, and the reimbursement is recorded as a recovery against that loss.
Sellers who book reimbursements to revenue get a double error, inflated sales and inventory still carrying units that no longer exist.
7. Applying a single average cost across a catalog
Signature: blended gross margin that looks fine while individual products are underwater.
Using one average cost across many items produces a total cost of goods sold that may be roughly correct while every per-product figure is wrong. Expensive items look more profitable than they are and cheap items look worse.
The statement-level total hides it. The damage happens in decisions: discontinuing a product that was actually carrying the catalog, or scaling one that loses money on every unit.
8. Changing valuation methods without documenting the change
Signature: a margin shift at a period boundary with no operational cause.
Moving between FIFO and weighted average changes reported cost of goods sold and therefore reported profit, without anything in the business changing. Done accidentally, usually as a side effect of new software defaulting differently, it produces a discontinuity nobody can explain.
Method changes generally require approval rather than a unilateral switch, and the rules on inventories and accounting methods are set out in IRS Publication 538. This is a decision to make with a CPA.
How to find which one you have
Three checks, in order.
Pull twelve months of gross margin by month and look for discontinuities. A step change points at method or categorization. Sawtooth swings point at purchase-timing expensing.
Then compare balance sheet inventory against a physical count including fulfillment centers and transit. A large gap points at errors three, four, or six.
Then take your five highest-volume items and rebuild their cost from scratch at true landed cost. If the rebuilt figure differs from what the system carries, you have error two or seven.
Most of this is preventable with a consistent period-close routine rather than an annual scramble. There is a working checklist for inventory reconciliation at period close at https://www.connectbooks.com/blog-posts/checklist-inventory-reconciliation-at-period-close for sellers who want a repeatable sequence.
For the broader financial management context, the Small Business Administration covers the fundamentals, and anything with a filing consequence belongs in front of an accountant who has worked with inventory businesses before.