Not all goods purchased in beginning & during the accounting period are sold until the end of that period, this results in a remainder balance known as closing stock.
Closing stock is a part of purchases & trial balance already includes purchases, hence if the closing stock is shown as a separate item it will double count and result in an error.
Example – Purchases for a period = 60,000, Closing Stock (remainder out of purchases) = 10,000, if both of these items are separately shown inside trial balance the effect will double up & trial balance will error out.
This one also stands tall among top finance and accounting interview questions asked in technical rounds by hiring managers.